[ADT] Add `zip_equal` for iteratees of equal lengths

Add a new version of `zip` that assumes that all iteratees have equal
lengths. The difference compared to `zip_first` is that `zip_equal`
checks this assumption in builds with assertions enabled.

This will allow us to clearly express the intent when working with
equally-sized ranges without having to write this assertion manually.

This is similar to Python's `zip(..., equal=True)` [1] or
`more_itertools.zip_equal` [2].

I saw this first suggested by @benvanik.

[1] https://peps.python.org/pep-0618/
[2] https://more-itertools.readthedocs.io/en/stable/api.html#more_itertools.zip_equal

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D138865