Resolves #11. Sub-circuits now selectable with ()

Circuits, at all language levels, can now be indexed by
step and depth.  The core method, @seq/subsref.m is fairly
well tested.  The language sequence and circuit methods are
 not as well tested, but seem solid after some ad hoc testing
 at the REPL.
v0.9.6 release. Circuits indexable by steps,depth

This release includes the feature discussed in issue