Fix deref interface in Fetch

We were only de-referencing pointers, but we can have pointer of
interface or interface obfuscating pointers...
So before trying to fetch anything, just deference fully the
variable.

expr_test.go

@@ -1204,6 +1204,21 @@ func TestExpr_fetch_from_func(t *testing.T) {
 	assert.Contains(t, err.Error(), "cannot fetch Value from func()")
 }
 
+func TestExpr_fetch_from_interface(t *testing.T) {
+	type FooBar struct {
+		Value string
+	}
+	foobar := &FooBar{"waldo"}
+	var foobarAny any = foobar
+	var foobarPtrAny any = &foobarAny
+
+	res, err := expr.Eval("foo.Value", map[string]any{
+		"foo": foobarPtrAny,
+	})
+	assert.NoError(t, err)
+	assert.Equal(t, "waldo", res)
+}
+
 func TestExpr_map_default_values(t *testing.T) {
 	env := map[string]any{
 		"foo": map[string]string{},

vm/runtime/runtime.go

@@ -8,6 +8,14 @@ import (
 	"reflect"
 )
 
+func deref(kind reflect.Kind, value reflect.Value) (reflect.Kind, reflect.Value) {
+	for kind == reflect.Ptr || kind == reflect.Interface {
+		value = value.Elem()
+		kind = value.Kind()
+	}
+	return kind, value
+}
+
 func Fetch(from, i any) any {
 	v := reflect.ValueOf(from)
 	kind := v.Kind()
@@ -28,10 +36,8 @@ func Fetch(from, i any) any {
 	// Structs, maps, and slices can be access through a pointer or through
 	// a value, when they are accessed through a pointer we don't want to
 	// copy them to a value.
-	if kind == reflect.Ptr {
-		v = reflect.Indirect(v)
-		kind = v.Kind()
-	}
+	// De-reference everything if necessary (interface and pointers)
+	kind, v = deref(kind, v)
 
 	// TODO: We can create separate opcodes for each of the cases below to make
 	// the little bit faster.