Add initial JSON formatter

It works and can format all "plain" Perl data, but has a few known bugs
and limitations that still need to be worked out.

It's about 8x smaller than JSON::XS's encoder and *much* smaller than
Cpanel::JSON::XS, but this is just a first attempt, it'll grow.

FU/XS.pm

@@ -1,3 +1,4 @@
+# This module is for internal use by other FU modules.
 package FU::XS 0.1;
 use Carp; # may be called by XS.
 use XSLoader;