Add `drop` as alias for `nthcdr`

Add `drop` as alias for `nthcdr`

It's a common name for this function and symmetric with respect to
`take`.  It's also a lot less awkward to type and say.

* lisp/subr.el (drop): New alias.
* doc/lispref/lists.texi (List Elements): Document.  Mention the
equation tying `take` and `drop` together.
* etc/NEWS: Announce.

(cherry picked from commit 8718a7c7062eb562e2fda0dee5f7bfddb0861f1d)