Infer state defaults from typing

[JamesArruda]

State defaults are inferred from types now, making valid_types unnecessary.

Closes #10

[JamesArruda]

Leaving this one in a failing state for now. Realizing with the tests that anything more than a top-level check of simple types is going to be a lot of effort. Even list[int] or tuple[int,...] kinds of things adds complexity I'm not sure is needed.

• Decide how far to take the type checking on state creation
• Decide if it should still happen on every assignment or just on initialization
• Split the MockEnvironment and other side things in this ticket off for a different MR.