More robust Nexus::name() method.

This version handles some error conditions.
Synthesis handle blocks of FF with unique CE

Like this:
  ... if (ce0) foo <= foo_in;
  ... if (ce1) bar <= bar_in;
Note that this is within a block, and represents multiple FF nodes
with different clock enables.
Handle casez without default case.

casez statements without default cases can get their input from
the context. This handles that situation properly.
Merge branch 'master' of github.com:steveicarus/iverilog