Add FU::XMLWriter

And remove UTF-8 check in JSON writer. It honestly feels kind of silly
to do that validation there while I've never done similar validations in
any other output routines - including this XML writer.

FU::XMLWriter is a copy of TUWF::XMLXS with a bunch of improvements
applied: now uses refcounts to determine the current output instance,
auto-generates XS functions and has faster escaped string output -
inspired by the JSON writer.

TODO:
- Integrate into FU
- Do something with bool attribute values
- Benchmarks
- Should $content be optional for all tags? The reason they weren't in
  TUWF::XMLXS is because TUWF::XML supports opening tags without closing
  them, but that idea turned out to suck and isn't supported anymore.

This is hopefully the last XS module for the FU framework. The only C
code being written now should be bug fixes and extending FU::Pg with
some planned features. Already ended up with more C than I had
planned...

c/jsonfmt.c

@@ -21,15 +21,6 @@ static void fujson_fmt_str(pTHX_ fujson_fmt_ctx *ctx, const char *stri, size_t l
     unsigned char *buf;
     unsigned char x = 0;
 
-    /* Validate entire string for conformance if this is flagged as a utf8
-     * string, this lets us be lazy further on.
-     * Commenting this out doubles the performance for formatting unicode
-     * strings, I suspect there's room for optimizations in
-     * is_c9strict_utf8_string().  */
-    if (utf8 && !is_c9strict_utf8_string(str, len)) {
-        croak("invalid codepoint encountered in string, cannot format to JSON");
-    }
-
     fustr_write_ch(ctx->out, '\"');
     fustr_reserve(ctx->out, len);
 
@@ -37,7 +28,7 @@ static void fujson_fmt_str(pTHX_ fujson_fmt_ctx *ctx, const char *stri, size_t l
         /* Fast path: no escaping needed */
         loff = off;
         if (utf8) {
-            /* we already validated everything >=0x80 */
+            /* assume >=0x80 is valid utf8 */
             while (off < len) {
                 x = str[off];
                 if (x <= 0x1f || x == '"' || x == '\\' || x == 0x7f) break;