rename 'constr_{fields,index}' to 'unconstr_{field,index}'

Better match the conventions so far used across the existing builtins.

examples/acceptance_tests/116/lib/tests.ak

@@ -1,12 +1,23 @@
-use aiken/builtin.{constr_index}
+use aiken/builtin.{unconstr_fields, unconstr_index}
 
-test baz() {
+test bar() {
   let x = Some("bar")
 
-  expect [bar] = x |> builtin.constr_fields
+  expect [bar] = x |> builtin.unconstr_fields
 
   and {
-    constr_index(x) == 0,
+    unconstr_index(x) == 0,
     builtin.un_b_data(bar) == "bar",
   }
 }
+
+test baz() {
+  let x = Some("baz")
+
+  expect [baz] = x |> unconstr_fields
+
+  and {
+    builtin.unconstr_index(x) == 0,
+    builtin.un_b_data(baz) == "baz",
+  }
+}