gnu: guile-fibers: Add patch to fix resource leak.

gnu: guile-fibers: Add patch to fix resource leak.

guile-fibers@1.0.0 has a resource leak where run-fibers will only destroy one
scheduler, but it creates as many as there are cpu cores by default (see
https://github.com/wingo/fibers/issues/36).  This causes the tests to fail on
systems with many cores, and can cause guile to crash under certain
circumstances.  This fixes that resource leak.  At present neither git master
nor the latest release has fixed this yet.

* gnu/packages/patches/guile-fibers-destroy-peer-schedulers.patch: New patch.
* gnu/local.mk: Add it to the list of patches.
* gnu/packages/guile-xyz.scm (guile-fibers): Use it.