SQUASHME: yamlcache: hide key calculation and fix public/private

Previously, key calculation had to be done manually, creating
unnecessary boilerplate. This has now been subsumed into YamlCache.get
and YamlCache.put (Yamlcache._get and Yamlcache._put_from_key exist for
old behaviour).

YamlCache.write is now private.

Private methods have been put at the end of the class.