Various type rep and type checker improvements

[jmanuel1]

• TODO: Remove generic type responsibility from this class [py_function] Take generics responsibility out of PythonFunctionType #41
• Separate overloaded type responsibility from py_function https://github.com/jmanuel1/concat/tree/separate-py-function-and-overloading
• Get mypy to accept comparison of kinds
  • FIXME: Not a total order, using total_ordering might be very unsound. [Kind]
• Represent mutually recursive types as fixpoints Represent mutually recursive classes as fixpoints #47
  • Distinguish between sequences (stacks), tuples, and variable arguments
• Rename _Function to StackEffect
• Use WHNF reduction Remove type objects from globals and reduce types to WHNF #56
• Use visitor pattern for constraints: Use the visitor pattern for checking constraints among types #58
• Mutable substitutions and vars? - Mutate a substitution in the context instead of applying immutable substitutions #59
• Internal intersection type?
  • Use it for subtyping?
• Split concat.typecheck.types into modules
• Move types defined in Python to stubs
• Use the argument types in py_function and make them accurate to Python, or remove them
• make type equality (is it really an equality?) a testing utility, because I think it's only used from tests
  • If equality is implemented separately from subsumption, use indices for alpha equivalence instead of substitutions

[jmanuel1]

https://typing.python.org/en/latest/spec/overload.html
https://typing.python.org/en/latest/spec/generics.html#paramspec
https://typing.python.org/en/latest/spec/callables.html
https://github.com/ponylang/ponyc/blob/main/src/libponyc/type/subtype.c#L612
https://repository.ubn.ru.nl/bitstream/handle/2066/145923/mmubn000001_194713903.pdf

[jmanuel1]

I think I need to support overloading of Python functions and Concat functions to type things like

• List.__getitem__
• arithmetic operators that support using __op__ on the first argument or __rop__ on the second argument

This overloading will be done based on the argument types, which means I need to track them.