Implement delindex!(x, idx)

[kmsquire]

#3023 (and #3405 and related issues) changed/added delete!, pop!, and splice!.

One unimplemented part of the proposal is the ability to delete an index from an array and return the modified array (rather than the deleted item or items).

It was suggested that delete! not be used for this, since it conflicts semantically with set deletion. delindex! was proposed, but never implemented.

One potential caveat mentioned is that this would not interact well with array views, although the same argument could be said of splice! and pop!.