instantiate type variables when assigned to a top-level binding

[kevinbarabash]

When generalizeType encounters a type variable outside of a function type it will generalize that type variable in the following way:

• if it has a default type associated with it, the default type will be used
• if it has a constraint/bound type associated with it, the constraint/bound type will be used
• if it has neither, unknown will be the type that the type var is generalized to

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 83.89%. Comparing base (899aa7c) to head (994c810).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #248 +/- ## ========================================== + Coverage 83.86% 83.89% +0.03% ========================================== Files 24 24 Lines 10372 10374 +2 Branches 1473 1472 -1 ========================================== + Hits 8698 8703 +5 + Misses 1674 1671 -3 ```

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