lib: add generator functions for toJSON & toYAML

They both reference the toJSON builtin, so we get semantic identifiers
that express the intent of the generation.
Both should be able to map each nix value (minus functions) to the
destination config files.

Includes two invocation unit tests.

lib/tests.nix

@@ -181,4 +181,27 @@ runTests {
     '';
   };
 
+  /* right now only invocation check */
+  testToJSONSimple =
+    let val = {
+      foobar = [ "baz" 1 2 3 ];
+    };
+    in {
+      expr = generators.toJSON {} val;
+      # trival implementation
+      expected = builtins.toJSON val;
+  };
+
+  /* right now only invocation check */
+  testToYAMLSimple =
+    let val = {
+      list = [ { one = 1; } { two = 2; } ];
+      all = 42;
+    };
+    in {
+      expr = generators.toYAML {} val;
+      # trival implementation
+      expected = builtins.toJSON val;
+  };
+
 }