Flip function access constraint generation

[JSAbrahams]

Relevant issues

Resolves #228

Summary

Now, the thing the function return type is being compared to must be the super. This makes sense, as a function may return a subset of the type its value is being assigned to, but not the other way around.

• Streamline env.get_var method. Doesn't make sense to have function argument in return.

Added Tests

• Unify access of a tuple arguments when tuple class attribute
• Tuple does not have type annotation when variable definition

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[JSAbrahams]

Well #228 appears to have been resolved, but you can't actually annotate tuples in Python so I'll have to modify the desugar stage.