0.3.13

This is a small release. It makes expensive generators more efficient with a
cache, allows rw and append tests a final chance to detect data loss at the end
of tests, and lets file snapshot nemeses run when /tmp is on a different
kind of filesystem.

New Features

• generator/cache: caches operations from an expensive generator to reduce
  (e.g.) IO costs.
• tests.cycle.rw and append now come with :final-generator and
  :wrap-generator which, during the final phase of a test, can issue final
  reads of all keys.
• snapshot-file-chunks now works across filesystem types.

Minor Changes

• core/prepare-test: Don't double-wrap generators in Forgettable when run as
  a part of test-all. This didn't break anything, but it was unnecesary.