Make RustBuiltin a scoped enum

This prevents name clashes. It already is used as scoped enum.

src/builtin.h

@@ -110,7 +110,7 @@ int parse_help_only_cmd_opts(help_only_cmd_opts_t &opts, int *optind, int argc,
                              const wchar_t **argv, parser_t &parser, io_streams_t &streams);
 
 /// An enum of the builtins implemented in Rust.
-enum RustBuiltin : int32_t {
+enum class RustBuiltin : int32_t {
     Abbr,
     Bg,
     Block,