Introduce `FieldLayout` abstraction to the C++ compiler.

This consolidates the field layout and hasbit assignment logic under a single abstraction, and refactors all code that interacts with either concept to use the field layout class.

Note that parse_function_generator can't depend on `FieldLayout` since the layout optimizers build a "virtual" fast parse table when determining field layout. We circumvent this problem by accepting a callable that returns the hasbit index of a field (all that the parse function generator needs).

PiperOrigin-RevId: 924929910