Reduce duplicated identifier strings in C parser and Ruby parser

[rmosolgo]

This PR dedups identifiers during parsing when parsing schemas.

A previous investigation (#4897) tried de-duping these in Schema members, but because AST nodes are retained along with the things that they define, these strings should be de-duplicated sooner, or else the originally-parsed strings will still be retained.

This greatly reduces retained strings in the benchmark:

  retained objects by class
  -----------------------------------
-     9897  String
      4978  Array
      4965  GraphQL::Language::Nodes::TypeName
      2745  Hash
      2340  GraphQL::Language::Nodes::FieldDefinition
      2340  GraphQL::Schema::Field
      2025  GraphQL::Language::Nodes::InputValueDefinition
      2025  GraphQL::Language::Nodes::NonNullType
      2025  GraphQL::Schema::Argument
       600  GraphQL::Schema::TypeMembership
       234  Class
+      164  String
       111  GraphQL::Schema::LateBoundType
       101  GraphQL::Language::Nodes::ObjectTypeDefinition
        10  GraphQL::Language::Nodes::UnionTypeDefinition
         7  Symbol

  allocated memory by gem
  -----------------------------------
-   22885432  graphql-ruby/lib
+   22879592  graphql-ruby/lib
-    2384584  graphql-c_parser/lib
+    2012304  graphql-c_parser/lib
      2080  set

I also updated the Ruby parser. It still allocates duplicate strings, but it doesn't retain them anymore:

- Total retained:  4756648 bytes (34386 objects)
+ Total retained:  4377104 bytes (24939 objects)

TODO:

• Address GC question with interned strings
• Make it so interning is on for schema parsing but off for query parsing
• Consider a similar patch for the Ruby parser?
• Using rb_enc_interned_str breaks Ruby 2.7 compat. Is this OK? (2.7 is EOL)