eval: Introduce a MapLike interface.

eval/map.go

@@ -10,6 +10,11 @@ type Map struct {
 	inner *map[Value]Value
 }
 
+type MapLike interface {
+	Lener
+	IndexOneer
+}
+
 // NewMap creates a new Map.
 func NewMap(inner map[Value]Value) Map {
 	return Map{&inner}

eval/struct.go

@@ -17,6 +17,11 @@ type Struct struct {
 	Fields     []Variable
 }
 
+var (
+	_ Value   = (*Struct)(nil)
+	_ MapLike = (*Struct)(nil)
+)
+
 func (*Struct) Kind() string {
 	return "map"
 }