[#61071] New features: VC timemachine and BackupOnSave to RCS

[John Yates <john@yates-sheets.org>]

New bug tracker issue #61071.  Here is [PATCH 0/3] cover letter:

This is a series of three bisectable (I hope :-) patches that culminate
in support of a new Emacs backup scheme:

* [PATCH 1/3] Refactor and document vc-find-revision caching
* [PATCH 2/3] Introduce VC timemachine capability
* [PATCH 3/3] Introduce vc-bos: backup on save (to an RCS file)

This Backup-On-Save scheme exploits a file system mirror scheme
introduced in the first patch.  By exploiting a little known aspect
of RCS's algorithm for locating a master file, backups are stored
completely removed from the work file (i.e. no local RCS directories)
and under exactly the same filename (i.e. no ',v' suffix or similar).

Accessing backed-ups exploits a new vc-timemachine capability,
introduced in the second patch.  Both the design and code owe much
to Peter Stiernström's original git-timemachine.el.  To sidestep any
copyright issues, Peter has graciously assigned git-timemachine.el's
copyright to the FSF.  With the submission timemachine functionality
is available in both vc-git and vc-rcs.

This backup scheme works equally well with files already under some
VCS as well as with files that are not currently version controlled.

For me (primarily a C++ programmer) this is:
  * My first significant bit of elisp
  * My first exposure to the VC codebase
  * My first Emacs / FSF submission

I welcome all nature of feedback:
  * Code criticism
  * Violations of pertinent standards
  * Bug reports
  * Suggested improvement
  * . . .