Typehints in core.py

[IgnaceBleukx]

Start of typehinting implementation.

Currently stuck on defining the "is_any_list" type, mypy is complaining about a lot it seems...
Something that's quite annoying is that we have explicit checks for which type a certain argument has (e.g., is_bool or is_num), but mypy of course has no idea these functions check what they say they check... so it complains even tho we know it is correct (there are "ignore" tags but they seem cumbersome)

[ThomSerg]

Using typing.cast we can give the required info to the static type checker. I included some small examples.
https://peps.python.org/pep-0484/#casts
It is an "empty function", so it will have a very very small performance impact.

Additionally, a lot of the code requires the args_list to be an "indexable", which Collection is not. The Sequence type would fix this (e.g. NestedList = Sequence[Union[T, "NestedList[T]"]]), but I believe this wouldn't allow for passing iterators. But doesn't seem to be a problem here, since we consume the iterator during construction (in the Expression.__init__).

In general I quite like this start.