CodeGenerator has multiple purposes.

First it's too batch `class_eval` calls, because each call has a fixed
cost , so by batching we're faster.

Second it's to be able to generate methods with the proper source
location. So that if a user introspect a method, it's pointed to
the proper macro.

And third, it is optionally to re-used generated method, typically
attribute accessors are very often duplicated.

Where it's used in Action View, there is no need for deduplication,
so `defined_cached_method` is just extra work.

Additionally, the source locations were incorrect because of the
use of heredoc.

This commit introduce `CodeGenerator#class_eval` for cases where
we don't need duplication, and fixes the source locations.