Add `unique_counts()`?

This is a follow-up of #275. I’ve found numpy.unique(x, return_counts=True) quite useful. It allows us to build a histogram for the input array x. I’d suggest us to add the following function:

def unique_counts(x, /):
   """
   return a tuple of two arrays (unique values and counts)
   """

[rgommers]

Seems very reasonable, and if we add it we should indeed add it to the v2021 milestone. Better to do all four new unique_* functions in one go.

[kgryte]

Thanks @leofang!