bug#24235: 25.1.50; `describe-bindings' request: show `??' definitions

[Drew Adams]

Enhancement request.  Keys shown by `describe-bindings' and similar
(including `substitute-command-keys' for \{...}) currently show `??' as
their binding if the binding is not a command symbol.

Please turn the `??' occurrences into links that when clicked show the
source code for the key's binding, when available.  If it is
byte-compiled, and the uncompiled code is unknown, show the byte code,
but with control chars filtered out (so somewhat human-readable).

This lets users see what is behind the `??', giving them a somewhat
better idea what the key does.

In GNU Emacs 25.1.50.1 (i686-pc-mingw32)
 of 2015-12-10
Repository revision: 6148555ee5a3d0139ae517803718b3e0357933c7
Windowing system distributor 'Microsoft Corp.', version 6.1.7601
Configured using:
 'configure --prefix=/c/Devel/emacs/snapshot/trunk --enable-checking=yes
 --enable-check-lisp-object-type --without-compress-install 'CFLAGS=-Og
 -ggdb3' LDFLAGS=-Lc:/Devel/emacs/lib 'CPPFLAGS=-DGC_MCHECK=1
 -Ic:/Devel/emacs/include''