Fix how we instantiate generic functions

[kevinbarabash]

This PR makes the following changes:

• type params without type args in generic function calls are instantiated as type (unification) variables now
• generation of dependency tree generation between type params has been extracted into a helper function
• instantiation of generic schemes has been update to use the new helper function

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 95.58824% with 3 lines in your changes missing coverage. Please review.

Please upload report for BASE (main@702ce47). Learn more about missing BASE report.

Files	Patch %	Lines
src/Escalier.TypeChecker/Unify.fs	66.66%	2 Missing :warning:
src/Escalier.TypeChecker/Poly.fs	97.87%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #250 +/- ## ======================================= Coverage ? 83.82% ======================================= Files ? 24 Lines ? 10327 Branches ? 1465 ======================================= Hits ? 8657 Misses ? 1670 Partials ? 0 ```

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