Add more complete support for vhdl local signals.

These signals are declared in the architecture and are local to
the module. The Architecture already parsed and stored these signal
declarations, but this patch adds the ability to actually emit these
signals in the generated code.

In the process of doing this, I had to regularize the elaboration
and emit of VTypes, so that it can be used in multiple places, not
just in entity headers (for ports).

I also added support for bit selects of signals. This effected a couple
places in the parser, and expressions in general.

vhdlpp/architec.cc

@@ -44,8 +44,8 @@ Architecture::Statement::~Statement()
 {
 }
 
-SignalAssignment::SignalAssignment(perm_string targ_name, list<Expression*>&rv)
-: target_name_(targ_name)
+SignalAssignment::SignalAssignment(ExpName*name, list<Expression*>&rv)
+: lval_(name)
 {
       rval_.splice(rval_.end(), rv);
 }