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

@@ -3,4 +3,22 @@
 #include "perl.h"
 #include "XSUB.h"
 
+#include "c/common.c"
+#include "c/jsonfmt.c"
+
 MODULE = FU   PACKAGE = FU::XS
+
+PROTOTYPES: DISABLE
+
+SV *json_format(val)
+    SV *val
+  PREINIT:
+    SV *r;
+    fustr buf = {};
+  CODE:
+    fujson_fmt(&buf, val);
+    r = fustr_sv(&buf);
+    SvUTF8_on(r);
+    RETVAL = r;
+  OUTPUT:
+    RETVAL