interp: fix use of builtins in type definitions

Make len() and cap() work on pointers. Preserve scope in case of
nested calls of cfg.

Fixes #1285.

interp/type.go

@@ -456,7 +456,7 @@ func nodeType2(interp *Interpreter, sc *scope, n *node, seen []*node) (t *itype,
 			length = int(vInt(sym.rval))
 		default:
 			// Size is defined by a numeric constant expression.
-			if _, err = interp.cfg(c0, sc.pkgID, sc.pkgName); err != nil {
+			if _, err = interp.cfg(c0, sc, sc.pkgID, sc.pkgName); err != nil {
 				return nil, err
 			}
 			v, ok := c0.rval.Interface().(constant.Value)