interp: fix closure in a struct field

Functions in a struct fields are always wrapped (as potentially
used by the runtime), so generate a function wrapper also for
closure when assigned to a struct field.

When such a function is called from the interpreter, ensure that
interface arguments are also wrapped so method and receiver resolution
can be performed.

Fixes partially #1043.

interp/type.go

@@ -1631,6 +1631,13 @@ func defRecvType(n *node) *itype {
 	return nil
 }
 
+func wrappedType(n *node) *itype {
+	if n.typ.cat != valueT {
+		return nil
+	}
+	return n.typ.val
+}
+
 func isShiftNode(n *node) bool {
 	switch n.action {
 	case aShl, aShr, aShlAssign, aShrAssign: