Refactor join arrayops and intidx internals

[nvictus]

This PR is attempting to simplify the logic and harmonize the internals of overlap and closest at all three layers (internal array function, internal intidx function, and public function):

• Change the low-level representation of the join output from a single (n, 2) array of indices to two (n,) arrays. This turns out to involve less intermediate manipulation and leads to more readable code.
• Streamline join type handling (left, right, outer) and better align overlap and closest implementation details.