Include which-key.el in the Emacs distribution

[Stefan Kangas <stefan@marxist.se>]

Dear all,

`which-key' is a package that unobtrusively shows you the available
keybindings at the bottom of the screen upon typing a prefix command
(after a configurable delay).  It is an excellent alternative and
complement to the default help.  See the screenshot here:

    https://raw.githubusercontent.com/justbur/emacs-which-key/master/img/which-key-bottom.png

`which-key' is in GNU ELPA, and was upgraded yesterday:

    https://elpa.gnu.org/packages/which-key.html

I propose to include `which-key' in the default Emacs distribution.
I expect that it will be extremely useful for beginning users, but it also
helps advanced users to quickly look up and discover keybindings.  In my
experience, that is true even in modes you already know deeply.

I've proposed this before, and have not seen any objections so far.
So I'd like to ask here again, to see if people have any concerns or
pointers for how to go about it in practice.

Any comments?

Best regards,
Stefan Kangas

PS. I personally think it should be enabled by default, but we would
    probably do better to first include it as an option for people to
    get more experience with it.