strip: move into core
As discussed at the 5.2 sprint, replace strip extension by a core
command, debugstrip. Obviously, the extension stays for backwards
compatibility.
As an implementation note, I moved the strip file as is into core,
which is not done elsewhere, AFAIK. I could have inlined it into
debugcommands, but that doesn't sound great.
Differential Revision:
https://phab.mercurial-scm.org/D9285
