interp: add support of interface wrappers to type assertions

Store the interpreter value of the interface object to wrap as
a field called IValue, at offset 0 in wrapper structures.

Update extract to include IValue field.

In typeAssert, detect interface wrapper, and dereference the
interpreter value from IValue wrapper field.

Fixes #1166.

interp/type.go

@@ -1176,6 +1176,9 @@ func (t *itype) id() (res string) {
 	case variadicT:
 		res = "..." + t.val.id()
 	}
+	if res == "" {
+		res = t.TypeOf().String()
+	}
 	return res
 }