Factor compile-time scopes into PScope class

Modules, functions and tasks are named scopes so derive them all
from the PScope base class. These items all take scoped items, so
the eventual plan is to move these items into PScope.

Module.cc

@@ -29,7 +29,7 @@
 
 /* n is a permallocated string. */
 Module::Module(perm_string n)
-: name_(n)
+: PScope(n)
 {
       library_flag = false;
       default_nettype = NetNet::NONE;