interp: fix support of interpreted functions in map values

Interpreted functions were represented in an inconsistent way in the frame: as a node pointer by default, and wrapped in a function wrapper for maps. 

We now simply use the default (*node) representation, as elsewhere, so values can be assigned, passed and called as for the other types. The alternative (generating a function wrapper) is more complex, costly and reserved for cases where the interpreted function can be called from the runtime.

Test that a map of functions can store both binary functions from used packages and interpreted ones.

Fixes #1090.

_test/map30.go

@@ -0,0 +1,22 @@
+package main
+
+import "strings"
+
+func f(s string) string { return "hello " + s }
+
+func g(s string) string { return "hi " + s }
+
+var methods = map[string]func(string) string{"f": f}
+
+func main() {
+	methods["i"] = strings.ToUpper
+	methods["g"] = g
+	println(methods["f"]("test"))
+	println(methods["g"]("test"))
+	println(methods["i"]("test"))
+}
+
+// Output:
+// hello test
+// hi test
+// TEST

interp/run.go

@@ -2349,7 +2349,7 @@ func mapLit(n *node) {
 	if n.nleft == 1 {
 		child = n.child[1:]
 	}
-	typ := n.typ.TypeOf()
+	typ := n.typ.frameType()
 	keys := make([]func(*frame) reflect.Value, len(child))
 	values := make([]func(*frame) reflect.Value, len(child))
 	for i, c := range child {
@@ -2384,7 +2384,7 @@ func compositeBinMap(n *node) {
 	if n.nleft == 1 {
 		child = n.child[1:]
 	}
-	typ := n.typ.TypeOf()
+	typ := n.typ.frameType()
 	keys := make([]func(*frame) reflect.Value, len(child))
 	values := make([]func(*frame) reflect.Value, len(child))
 	for i, c := range child {

interp/type.go

@@ -1554,6 +1554,8 @@ func (t *itype) frameType() (r reflect.Type) {
 			break
 		}
 		r = reflect.TypeOf((*valueInterface)(nil)).Elem()
+	case mapT:
+		r = reflect.MapOf(t.key.frameType(), t.val.frameType())
 	case ptrT:
 		r = reflect.PtrTo(t.val.frameType())
 	default: