captures for "when" statements

update Triggers to IntrusivePtr's and simpler AST traversal
introduce IDSet type, migrate associated "ID*" types to "const ID*"

src/Val.h

@@ -82,6 +82,7 @@ class TypeVal;
 
 using AddrValPtr = IntrusivePtr<AddrVal>;
 using EnumValPtr = IntrusivePtr<EnumVal>;
+using FuncValPtr = IntrusivePtr<FuncVal>;
 using ListValPtr = IntrusivePtr<ListVal>;
 using PortValPtr = IntrusivePtr<PortVal>;
 using RecordValPtr = IntrusivePtr<RecordVal>;