Here's a first draft of a new union.scm.

On my YeeLoong 8101B, when building my 155-package profile, it uses
about 1/14 as much CPU time (25 seconds vs 6 minutes), and about 2/7 as
much real time (2 minutes vs about 7.17 minutes).  It also handles
libffi in core-updates properly.  A few notes:

* For now, I'm using a hash table, because it turned out to be rather
  awkward to use a vhash for this.  I can make it purely functional
  later if it's deemed important.

* I've not yet updated tests/union.scm, which tests internal procedures
  in union.scm that no longer exist.

* Although I can now run union-build in 2 minutes, another full minute
  is spent in 'display-search-paths', plus another 30-40 seconds in
  'guix package' before the profile build begins.

In the end, the total time for "guix package -i" goes from around 8.5
minutes to around 3.5 minutes: a significant improvement, but there's
still more code for me to look at trying to optimize.

Instead of a patch, I've simply attached the new union.scm.  The only
code they have in common is 'file=?', which is unchanged.

Comments and suggestions welcome,

      Mark