Don't turn constant names into strings prematurely

This pull request changes how `ActiveSupport::Dependencies#new_constants_in` handles constant names internally. 

While it still returns strings so that assumptions of calling code aren't affected, it will now use symbols to represent constant names within the method and only converts them to strings on return.

This shaves about 300ms off app boot time, and should make dev reloading a little bit faster as well.

cc @github/perf