Various type rep 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
  • [-] Distinguish between sequences (stacks), tuples, and variable arguments
• Rename _Function to StackEffect
• Use WHNF reduction
• Use visitor pattern for constraints
• Mutable substitutions and vars?
• Other todos
  • If equality is implemented separately from subsumption, use indices for alpha equivalence instead of substitutions
• 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