Convert var_table_t to an unordered_map

Profiling revealed string comparison in variable lookups to be a
significant hotspot. This change causes `make test` to complete ~4.5%
faster per `hyperfine`.

src/env.cpp

@@ -635,7 +635,7 @@ std::shared_ptr<const null_terminated_array_t<char>> env_scoped_impl_t::create_e
             assert(var && "Variable should be present in uvars");
             // Note that std::map::insert does NOT overwrite a value already in the map,
             // which we depend on here.
-            vals.insert(std::make_pair(key, *var));
+            vals.insert(std::move(std::make_pair(std::move(key), std::move(*var))));
         }
     }
 

src/env.h

@@ -5,9 +5,9 @@
 #include <stddef.h>
 #include <stdint.h>
 
-#include <map>
 #include <memory>
 #include <string>
+#include <unordered_map>
 #include <vector>
 
 #include "common.h"
@@ -186,7 +186,7 @@ class env_var_t {
     }
     bool operator!=(const env_var_t &rhs) const { return !(*this == rhs); }
 };
-typedef std::map<wcstring, env_var_t> var_table_t;
+typedef std::unordered_map<wcstring, env_var_t> var_table_t;
 
 /// An environment is read-only access to variable values.
 class environment_t {

src/env_universal_common.cpp

@@ -441,7 +441,14 @@ std::string env_universal_t::serialize_with_vars(const var_table_t &vars) {
     contents.append(SAVE_MSG);
     contents.append("# VERSION: " UVARS_VERSION_3_0 "\n");
 
-    for (const auto &kv : vars) {
+    // Preserve legacy behavior by sorting the values first
+    typedef std::pair<std::reference_wrapper<const wcstring>, std::reference_wrapper<const env_var_t>> env_pair_t;
+    std::vector<env_pair_t> cloned(vars.begin(), vars.end());
+    std::sort(cloned.begin(), cloned.end(), [](const env_pair_t &p1, const env_pair_t &p2) {
+        return p1.first.get() < p2.first.get();
+    });
+
+    for (const auto &kv : cloned) {
         // Append the entry. Note that append_file_entry may fail, but that only affects one
         // variable; soldier on.
         const wcstring &key = kv.first;