feat: Generic function definitions

[mark-koch]

Allow function definitions that are generic over parameters of kind type or nat.

• Store currently available type parameters in the type checking context
• Refactor type parsing logic to explicitly opt in to add free variables to the parameter mapping
• Add a new GenericParamValue AST node to represent usages of generic nat params n inside function bodies. We need a way to encode this in Hugr. For now, we just emit a dummy node. See https://github.com/CQCL/hugr/issues/1629

Note: Nested generic functions are not supported yet, so we don't have to worry about scoping of type params.

Closes #522

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 90.74074% with 5 lines in your changes missing coverage. Please review.

Project coverage is 92.61%. Comparing base (f527703) to head (5efa146).

Files with missing lines	Patch %	Lines
guppylang/checker/expr_checker.py	84.61%	2 Missing :warning:
guppylang/tys/parsing.py	85.71%	2 Missing :warning:
guppylang/tys/printing.py	66.66%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #618 +/- ## ========================================== + Coverage 92.45% 92.61% +0.15% ========================================== Files 66 66 Lines 7465 7487 +22 ========================================== + Hits 6902 6934 +32 + Misses 563 553 -10 ```

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