draft: first pass at overloading verb_dispatch

[machow]

Addresses #465, #418.

• Annotates the return type for verb_dispatch as functools._SingleDispatchCallable, so it reflects the behavior of something wrapped in @singledispatch.

Need to remove pylance / mypy type checking errors for code like this:

from siuba import mutate

# Pylance: Expected 1 more positional argument
mutate(a = 1)

@singledispatch
def f(a, b): ...

# No type checking error
f()

[machow]

Alright, adding an explicit type for the cls argument worked. Going to quickly fix the ci build and then will merge in the next day or so!

[machow]

I use VS Code and type hints a lot now, and have had to deal with situations like this a bit more, so am going to close and take a better pass at this!