builtins: Reduce streams.out.append/push_back calls

This basically immediately issues a "write()" if it's to a pipe or the
terminal.

That means we can reduce syscalls and improve performance, even by
doing something like

```c++
streams.out.append(somewcstring + L"\n");
```

instead of

```c++
streams.out.append(somewcstring);
streams.out.push_back(L'\n');
```

Some benchmarks of the

```fish
for i in (string repeat -n 2000 \n)
    $thing
end
```

variety:

1. `set` (printing variables) sped up 1.75x
2. `builtin -n` 1.60x
3. `jobs` 1.25x (with 3 jobs)
4. `functions` 1.20x
5. `math 1 + 1` 1.1x
6. `pwd` 1.1x

Piping yields similar results, there is no real difference when
outputting to a command substitution.

src/builtins/builtin.cpp

@@ -101,11 +101,8 @@ maybe_t<int> builtin_builtin(parser_t &parser, io_streams_t &streams, const wcha
         wcstring_list_t names = builtin_get_names();
         std::sort(names.begin(), names.end());
 
-        for (const auto &name : names) {
+        for (auto &name : names) {
-            auto el = name.c_str();
+            streams.out.append(name + L"\n");
-
-            streams.out.append(el);
-            streams.out.append(L"\n");
         }
     }
 

src/builtins/functions.cpp

@@ -286,9 +286,8 @@ maybe_t<int> builtin_functions(parser_t &parser, io_streams_t &streams, const wc
 
             streams.out.append(reformat_for_screen(buff, termsize_last()));
         } else {
-            for (const auto &name : names) {
+            for (auto &name : names) {
-                streams.out.append(name.c_str());
+                streams.out.append(name + L"\n");
-                streams.out.append(L"\n");
             }
         }
 

src/builtins/jobs.cpp

@@ -46,6 +46,7 @@ static void builtin_jobs_print(const job_t *j, int mode, int header, io_streams_
         pgid = *job_pgid;
     }
 
+    wcstring out;
     switch (mode) {
         case JOBS_PRINT_NOTHING: {
             break;
@@ -53,53 +54,57 @@ static void builtin_jobs_print(const job_t *j, int mode, int header, io_streams_
         case JOBS_DEFAULT: {
             if (header) {
                 // Print table header before first job.
-                streams.out.append(_(L"Job\tGroup\t"));
+                out.append(_(L"Job\tGroup\t"));
                 if (have_proc_stat()) {
-                    streams.out.append(_(L"CPU\t"));
+                    out.append(_(L"CPU\t"));
                 }
-                streams.out.append(_(L"State\tCommand\n"));
+                out.append(_(L"State\tCommand\n"));
             }
 
-            streams.out.append_format(L"%d\t%d\t", j->job_id(), pgid);
+            append_format(out, L"%d\t%d\t", j->job_id(), pgid);
 
             if (have_proc_stat()) {
-                streams.out.append_format(L"%.0f%%\t", 100. * cpu_use(j));
+                append_format(out, L"%.0f%%\t", 100. * cpu_use(j));
             }
 
-            streams.out.append(j->is_stopped() ? _(L"stopped") : _(L"running"));
+            out.append(j->is_stopped() ? _(L"stopped") : _(L"running"));
-            streams.out.append(L"\t");
+            out.append(L"\t");
-            streams.out.append(j->command_wcstr());
+            out.append(j->command_wcstr());
-            streams.out.append(L"\n");
+            out.append(L"\n");
+            streams.out.append(out);
             break;
         }
         case JOBS_PRINT_GROUP: {
             if (header) {
                 // Print table header before first job.
-                streams.out.append(_(L"Group\n"));
+                out.append(_(L"Group\n"));
             }
-            streams.out.append_format(L"%d\n", pgid);
+            append_format(out, L"%d\n", pgid);
+            streams.out.append(out);
             break;
         }
         case JOBS_PRINT_PID: {
             if (header) {
                 // Print table header before first job.
-                streams.out.append(_(L"Process\n"));
+                out.append(_(L"Process\n"));
             }
 
             for (const process_ptr_t &p : j->processes) {
-                streams.out.append_format(L"%d\n", p->pid);
+                append_format(out, L"%d\n", p->pid);
             }
+            streams.out.append(out);
             break;
         }
         case JOBS_PRINT_COMMAND: {
             if (header) {
                 // Print table header before first job.
-                streams.out.append(_(L"Command\n"));
+                out.append(_(L"Command\n"));
             }
 
             for (const process_ptr_t &p : j->processes) {
-                streams.out.append_format(L"%ls\n", p->argv0());
+                append_format(out, L"%ls\n", p->argv0());
             }
+            streams.out.append(out);
             break;
         }
         default: {

src/builtins/math.cpp

@@ -252,8 +252,7 @@ static int evaluate_expression(const wchar_t *cmd, const parser_t &parser, io_st
             streams.err.append_format(L"'%ls'\n", expression.c_str());
             retval = STATUS_CMD_ERROR;
         } else {
-            streams.out.append(format_double(v, opts));
+            streams.out.append(format_double(v, opts) + L"\n");
-            streams.out.push_back(L'\n');
         }
     } else {
         streams.err.append_format(L"%ls: Error: %ls\n", cmd, math_describe_error(error));

src/builtins/path.cpp

@@ -201,8 +201,7 @@ static void path_out(io_streams_t &streams, const options_t &opts, const wcstrin
         if (!opts.null_out) {
             streams.out.append_with_separation(str, separation_type_t::explicitly);
         } else {
-            streams.out.append(str);
+            streams.out.append(str + L"\0");
-            streams.out.push_back(L'\0');
         }
     }
 }

src/builtins/pwd.cpp

@@ -73,7 +73,6 @@ maybe_t<int> builtin_pwd(parser_t &parser, io_streams_t &streams, const wchar_t
     if (pwd.empty()) {
         return STATUS_CMD_ERROR;
     }
-    streams.out.append(pwd);
+    streams.out.append(pwd + L"\n");
-    streams.out.push_back(L'\n');
     return STATUS_CMD_OK;
 }

src/builtins/set.cpp

@@ -437,7 +437,8 @@ static int builtin_set_list(const wchar_t *cmd, set_cmd_opts_t &opts, int argc,
     sort(names.begin(), names.end());
 
     for (const auto &key : names) {
-        streams.out.append(key);
+        wcstring out;
+        out.append(key);
 
         if (!names_only) {
             wcstring val;
@@ -460,14 +461,15 @@ static int builtin_set_list(const wchar_t *cmd, set_cmd_opts_t &opts, int argc,
                     shorten = true;
                     val.resize(60);
                 }
-                streams.out.append(L" ");
+                out.push_back(L' ');
-                streams.out.append(val);
+                out.append(val);
 
-                if (shorten) streams.out.push_back(get_ellipsis_char());
+                if (shorten) out.push_back(get_ellipsis_char());
             }
         }
 
-        streams.out.append(L"\n");
+        out.push_back(L'\n');
+        streams.out.append(out);
     }
 
     return STATUS_CMD_OK;