Denotability for mutations

#412 would benefit from denotability of mutations within functions:

```py
a = 0

@Mutates('a')
def foo():
global a
a = 1
```

This would help in stubs and in inheritance.

Start by reading issue #412 and the mutation example in this issue. No files, tests, or entry points are named, so identify where the type checker handles Mutates annotations and function-scope global assignments. Done means mutations can be denoted within functions for stubs and inheritance.

Written by the indexing model from the issue text.