interp: fix detection of binary types implementing interfaces

Fixes #1136.

interp/type.go

@@ -1558,7 +1558,7 @@ func (t *itype) frameType() (r reflect.Type) {
 }
 
 func (t *itype) implements(it *itype) bool {
-	if t.cat == valueT {
+	if isBin(t) {
 		return t.TypeOf().Implements(it.TypeOf())
 	}
 	return t.methods().contains(it.methods())
@@ -1730,6 +1730,17 @@ func isInterface(t *itype) bool {
 	return isInterfaceSrc(t) || t.TypeOf() != nil && t.TypeOf().Kind() == reflect.Interface
 }
 
+func isBin(t *itype) bool {
+	switch t.cat {
+	case valueT:
+		return true
+	case aliasT, ptrT:
+		return isBin(t.val)
+	default:
+		return false
+	}
+}
+
 func isStruct(t *itype) bool {
 	// Test first for a struct category, because a recursive interpreter struct may be
 	// represented by an interface{} at reflect level.