replacing ldefs-boot.el

[Phillip Lord]

I've been working on a patch to replace ldefs-boot.el.

ldefs-boot.el is necessary for Emacs to build from temacs to bootstrap,
and works by providing all the autoloads necessary for Emacs to build to
bootstrap and then compile all the COMPILE_FIRST sources.

Currently, ldefs-boot.el is just a copy of loaddefs.el, copied into the
repo regularly. It's a large, generated file and has to be in the repo
(or else you would need a copy from somewhere else to build Emacs).

It works by providing a much smaller file which provides just the
autoloads necessary to build to bootstrap, and then generates
loaddefs.el before it COMPILE_FIRST's. This file can also be regenerated
automatically, although I suspect that this will be a rare occurence.

It's not 100% finished yet (some comments need cleaning up), but I'd
like in-principle agreement for this to go to master before I do that.

The code is on feature/manual-ldefs-rebased

Phil