add convenience wrapper for array updating with fancy index expressions

[mattjj]

We should provide an update function that lets users write

A = lax.update(A, (slice(1, None, 2), Ellipsis, slice(None, -3)), value)

which would behave similarly to this kind of expression in regular NumPy

A[1::2, ...,  :-3] = value

except without the in-place mutation that would update any aliases.

This wrapper would be similar to the lax_numpy._rewriting_take function we already have for handling complex indexing expressions, and would ultimately call into primitives like lax.dynamic_update_slice and lax.index_untake. Better name suggestions welcome!

This is a separate issue from whether to provide more convenient syntax for this kind of operation.

[georgedahl]

This is a really important operation for making it simple to express certain models. When using TF I have often had to spend a lot of time figuring out exactly how to use scatter_nd or whatever so making a clear correspondence with numpy syntax is useful.