Add some support for procedural $signed/$unsigned.

This patch adds support for finding and adding $signed() and $unsigned()
to procedural expressions for the binary, unary and concatenation
operators. Selects have partial support. The select implementation
uncovered limitations in the compiler that need to be fixed before they
will work 100%. Most of the limitations currently generate a message
about the issue.
Merge branch 'master' of git://github.com/steveicarus/iverilog