This reduces memory and CPU usage (unless you only have one CPU core),
and reduces the time needed to compile, during 'guix pull'.


Memory:

It used to use as many guile processes as you have cores, each consuming
about 100-150 MB memory.  Now it uses a single guile process whose
memory consumption grows linearly until it reaches almost 300 MB.


CPU:

It used to max out every CPU core, now just one. :-)


Time:

It used to take ~18 minutes on my machine, now less than 3.


The code is vastly simplified as well, as you see.  I'm not sure if
there's a good way to parallelize it though, and it's somewhat hacky to
keep all the modules in memory instead of spawning a clean guile to
compile each file...