subr-x on Elpa

[Artur Malabarba]

subr-x has a ton of useful macros and functions, and it would be nice
if it were available to packages that support emacs versions down to
24.1. So I'd like to make it into a proper package and put it on GNU
Elpa, if possible.

AFAICT, the only place where subr-x won't work with 24.1 is in the
`when-let' macro, which uses `macroexp-progn'. But `macroexp-progn' is
a very simple function, so inlining its definition is trivial.
If I do that, all of the tests in subr-x-tests.el pass with Emacs 24.1
(except for one, which explicitly uses cl-reduce, but that has nothing
to do with the function being tested, and even that one passes if I
load cl-lib too).

If I do that, would it be viable to offer subr-x on GNU Elpa as well?
(of course it would not be removed from lisp/emacs-lisp dir)