In my lurkings I've seen people wondering why `guix environment`
doesn't work with manifests. The answer is simply: I never thought to
add it. This patch fixes that. The implementation is kind of
interesting and might seem a little silly to people that know how
manifests work.  In order to support manifests with minimal code and
make --manifest compose with other options I simply decompile the
manifest back into a list of package/output tuples.  That means in the
case of `guix environment --manifest=foo.scm` the manifest is created,
decompiled, and a new manifest created from that.  Seems redundant!
The advantage is that since --manifest composes with all the other
ways to specify packages we can do absolutely bonkers things like
`guix environment guile --ad-hoc ruby --manifest=foo.scm
--manifest=bar.scm --load=frob.scm --expression='(@ (gnu packages
python) python)'`.  More realistically you'd use it to throw in an
extra package or two with --ad-hoc.

Anyway, hope y'all like it.

- Dave