rmeta-slot: New module for recursive meta-slot access.

This is an abstraction around the core idea driving the actor actions
system.  It turns out I need this abstraction for mudsync as well, so
I'm breaking it out.  Pleasantly, it should be much faster now, since
a cache is built for each class regarding what key should resolve to
what value.

* 8sync/rmeta-slot.scm:
* tests/test-rmeta-slot.scm: New modules.
* Makefile.am: Add them.