Add type-hints to expressions

[ciortiz]

An additional tool for documenting callable expressions is to add type-hints, which map arguments or the output of the expression to the corresponding type, e.g.:

def add_numbers(a: int | float, b: int | float) -> int | float:
return a + b

This would be useful for new contributors or users, in learning about the expectations of an expression, and for testing purposes, if additional checks were to be made to ensure that expressions behave as expected.

This issue is more-so targeted for code that has been already written. New code should feature type-hints.