jsonfmt: Move arg parsing into XS

Going to need a way to pass arguments into the XS function anyway, so
might as well do the entire arg parsing step in XS while we're at it.
Provides a significant speedup for tiny inputs as well, but I don't find
that too interesting.

c/jsonfmt.c

@@ -172,6 +172,35 @@ static void fujson_fmt(pTHX_ fustr *out, SV *val) {
     }
 }
 
+
+static SV *fujson_fmt_xs(pTHX_ I32 ax, I32 argc, SV *val) {
+    I32 i = 1;
+    int encutf8 = 0;
+    char *arg;
+    SV *r;
+
+    while (i < argc) {
+        arg = SvPV_nolen(ST(i));
+        i++;
+        if (i == argc) croak("Odd name/value argument for json_format()");
+        r = ST(i);
+        i++;
+
+        if (strcmp(arg, "utf8") == 0) {
+            encutf8 = SvPVXtrue(r);
+        } else {
+            croak("Unknown flag: '%s'", arg);
+        }
+    }
+
+    fustr buf;
+    fustr_init(&buf, 128);
+    fujson_fmt(aTHX_ &buf, val);
+    r = fustr_done(&buf);
+    if (!encutf8) SvUTF8_on(r);
+    return r;
+}
+
 /* TODO: canonical */
 /* TODO: pretty */
 /* TODO: max depth? */