vlog95: Add $signed/$unsigned support for signals

The actual signal code is trivial, but the emit expression code needed
to be enhanced to pass a flag that says if one of the arguments in a
binary (except the shifts) or ternary (excluding the condition) context
is unsigned. This information is used to prevent emitting an explicit
$unsigned() for a signal that is used in this context since it will be
implicitly cast to unsigned.