Split up trait_types.py

Th file is physically very large and could easily be broken up into a series of modules (or a subpackage with a series of modules) each of which is more focused (eg. a module with `BaseInt`, `Int`, etc., another with `Str` etc., another with `Property` and related code, and so on...)

For backwards compatibility `trait_types.py` may need to remain as a stub for some amount of time.