[MOREL-27] Create objects for built-in structures List and String…

…, and allow `structure.name` references

Previously we had values structure_name, for example `List_hd`
and `List_tl`; these are now replaced with a value for each
structure.  For example value `List` has fields `hd`, `tl`.
Similarly `String`, `Sys`, `Relational`.

src/main/java/net/hydromatic/morel/ast/AstBuilder.java

@@ -407,8 +407,19 @@ public Ast.Aggregate aggregate(Pos pos, Ast.Exp aggregate, Ast.Exp argument,
     return new Ast.Aggregate(pos, aggregate, argument, id);
   }
 
+  /** Returns a reference to a built-in: either a name (e.g. "true")
+   * or a field reference (e.g. "#hd List"). */
+  private Ast.Exp ref(Pos pos, BuiltIn builtIn) {
+    if (builtIn.structure == null) {
+      return id(pos, builtIn.mlName);
+    } else {
+      return apply(id(pos, builtIn.structure),
+          id(pos, builtIn.mlName));
+    }
+  }
+
   public Ast.Exp map(Pos pos, Ast.Exp e1, Ast.Exp e2) {
-    return apply(apply(id(pos, BuiltIn.LIST_MAP.mlName), e1), e2);
+    return apply(apply(ref(pos, BuiltIn.LIST_MAP), e1), e2);
   }
 
   public Ast.Order order(Pos pos, Iterable<Ast.OrderItem> orderItems) {