This patch addresses the issue of how to ensure that Ruby executables
are able to load all of the additional Ruby libraries that they need in
order to work: with a new 'patch-executables' build phase.  Instead of
using wrap-program, I instead take advantage of the wrappers that the
'gem' command already creates for Ruby executables.  There's guaranteed
to be a line in which the executable's host gem is loaded that looks
like `gem 'foo', version`.  I simply insert a Ruby code snippet above it
that adds all of the necessary gems to the 'Gem.path' array.

Users of Ruby programs must still apply the $GEM_PATH suggested by 'guix
package --search-paths' in order for the gem that the executable belongs
to (the gem that they explicitly installed) to be found.  I think this
is reasonable and much like how you must set the proper load paths for
Guile programs to work.