[types] Add module for type validation

[Ableytner]

Adds a new module (abllib.types) which can validate types, raising a WrongTypeError on mismatch.

This adds the following new functions:

• abllib.types.enforce_var: enforces the type of a given value
• abllib.types.enforce_args: enforces the type of all function arguments and the return value
• abllib.types.enforce: a combined function usable instead of the functions above

TODO before merge:

• add support for recursive type hints (see _PersistentStorage.__setitem__)