Use type param defaults when there aren't enough type args

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 75.00000% with 7 lines in your changes missing coverage. Please review.

Project coverage is 83.81%. Comparing base (b400221) to head (e4b392c).

Files	Patch %	Lines
src/Escalier.TypeChecker/Unify.fs	75.00%	7 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #288 +/- ## ======================================= Coverage 83.80% 83.81% ======================================= Files 23 23 Lines 10760 10785 +25 Branches 1537 1543 +6 ======================================= + Hits 9017 9039 +22 - Misses 1743 1746 +3 ```

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