`_code_from_builtin_name` (with the leading underscore) now has native string literals, i.e. 8-bit strings on Python 2.

`code_from_builtin_name` (without leading underscore) has the contents converted to unicode strings. We could have just defined this directly, but the mass of `UNICODE_LITERAL()` calls would have been ugly, so I chose to build it like this and then remove `_code_from_builtin_name`.