Improved interface for c_enum_typet

Interface for c_enum_typet has limited support for management of enum
members.
This commit adds
- a function for exposing the member vector as a non-const reference,
- a constructor that takes the underlying type and a list of members and
  sets the constructed type accordingly.

Author: Enrico Steffinlongo

src/util/c_types.h

@@ -274,6 +274,17 @@ class c_enum_typet : public type_with_subtypet
 
   typedef std::vector<c_enum_membert> memberst;
 
+  c_enum_typet(typet _subtype, memberst enum_members)
+    : c_enum_typet(std::move(_subtype))
+  {
+    members() = std::move(enum_members);
+  }
+
+  memberst &members()
+  {
+    return (memberst &)(add(ID_body).get_sub());
+  }
+
   const memberst &members() const
   {
     return (const memberst &)(find(ID_body).get_sub());