Make array correspondence simpler, slightly safer, and make API more numpy-like

[tovacinni]

This new array correspondence makes:

1. The API more numpy-like. The axis now specifies the dimension to do the correspondence search over.
2. Adds some asserts for safety.
3. Some simple comments to explain what the code is doing.
4. Replace the byte comparison (which will amount to pointer comparisons to objects) with multi-dim np.unique which is a bit more readable and works for single-dim object arrays.

Unfortunately np.unique doesn't work well for multi-dim object arrays... but these would've failed in the any case in the older version so at least it's not a regression.

[odedstein]

@tovacinni I changed a few things. Please tell me whether you are ok with them (I'm still working on the failing unit tests).
Since we have a breaking change with this PR (the axis convention), we'll try to wait for the next 0.x.0 version to put this in main.

[tovacinni]

Sounds good with me!