ENH: add new function isin

[OmarManzoor]

Add support for the isin function

towards gh-268

[OmarManzoor]

@lucascolley I tried adding isin. I think there might be adjustments required particularly in the delegation part and also more tests but maybe you could have a look first to see if this generally looks okay.

[lucascolley]

thanks @OmarManzoor, looks good to me overall! The delegation strategy makes sense to me.

For the failing NumPy 1 tests, probably fine to just skip.

[lucascolley]

thanks @OmarManzoor, looks great!

[lucascolley]

@all-contributors please add @OmarManzoor for docs, bug, test

[allcontributors]

@lucascolley

I've put up a pull request to add @OmarManzoor! 🎉

[OmarManzoor]

@lucascolley Should we add in fill_diagonal as well?

[lucascolley]

I can't find any discussion about it in the array-api repo. So yes, probably good to add it here, but could you do some research as to which libraries provide an implementation first?