Allow external code using libnixexpr to add types

Code that links to libnixexpr (e.g. plugins loaded with importNative, or
nix-exec) may want to provide custom value types and operations on
values of those types. For example, nix-exec is currently using sets
where a custom IO value type would be more appropriate. This commit
provides a generic hook for such types in the form of tExternal and the
ExternalBase virtual class, which contains all functions necessary for
libnixexpr's type-polymorphic functions (e.g. `showType`) to be
implemented.

src/libexpr/primops.cc

@@ -187,6 +187,9 @@ static void prim_typeOf(EvalState & state, const Pos & pos, Value * * args, Valu
         case tPrimOpApp:
             t = "lambda";
             break;
+	case tExternal:
+            t = args[0]->external->typeOf();
+            break;
         default: abort();
     }
     mkString(v, state.symbols.create(t));