This adds a first version of a functional vim-build-system.

As explained today on IRC, and as you can guess by reading
gnu/packages/vim.scm Vim has a concept of folders and files in
standard places, but Makefiles and similar tools are an
exception. Usually you have to copy them somewhere (or use
external Vim package managers that pull the files for yoz and set
the "run time path" (rtp) for you).

There's also (to my best knowledge, keep in mind I've last read
into Vim when I did the last commits in the Vim module) no ENV
VAR or anything we could export, you have to manually set

  set rtp+=~/.guix-profile/share/vim/vimfiles/

in your $home/.vimrc

The build system comes with a FIXME note, but it works. I'm in
the middle of tests and exams, so this is the best I can correct
from my earlier attempt in '17 on this. What I can apply are
changes and adjustments to what I submit now.

4 Patches attached, 3 of them are for proof of work.