ENH: delegation tracker

[lucascolley]

Where native implementations (more simple than the implementations in _funcs.py) are possible for known array library backends, we should add delegation to those implementations, instead of using the array-agnostic implementation. This is for performance reasons, and it may also allow us to overcome limitations in the "agnostic" implementations if they are not fully agnostic.

See https://data-apis.org/array-api-extra/contributing.html#how-to-add-delegation-to-a-function for how to do this.

• atleast_nd
• cov
• create_diagonal
• expand_dims
• kron
• nunique
• pad
• setdiff1d
• sinc