Add checks preventing assigning values on defining types.

[jacqueswww]

- What I did

Fixes #410 adds tests.

- How I did it

Figured out it has to do with .value attribute on annotate assign.

- How to verify it

The following will fail to compile now:

def foo() -> num:
    q:num = 111
    return q

- Description for the changelog

Fixes ambiguous assign on annotate syntax.

- Cute Animal Picture

[DavidKnott]

LGTM