shell: Maintain a profile cache.

shell: Maintain a profile cache.

With this change, running "guix shell" (no arguments) is equivalent to:

  guix environment -r ~/.cache/guix/profiles/some-root -l guix.scm

This is the cache miss.  On cache hit, it's equivalent to:

  guix environment -p ~/.cache/guix/profiles/some-root

... which can run in 0.1s.

* guix/scripts/shell.scm (options-with-caching): New procedure.
(parse-args): Use it.
(%profile-cache-directory): New variable.
(profile-cache-key, profile-cached-gc-root): New procedures.
(show-help, %options): Add '--rebuild-cache'.
(guix-shell)[cache-entries, entry-expiration]: New procedures.
Add call to 'maybe-remove-expired-cache-entries'.
* doc/guix.texi (Invoking guix shell): Document '--rebuild-cache'.

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