Add associative map constraints and range refinement

examples/map-ascription.dcdl

@@ -0,0 +1,15 @@
+let
+    Service = {
+        port = Int & >= 1 & <= 65535;
+        enabled = Bool default true;
+    };
+in
+    {
+        api = {
+            port = 8080;
+        };
+        worker = {
+            port = 8081;
+            enabled = false;
+        };
+    } as {...Service}