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...

FU/Util.pm

@@ -163,6 +163,10 @@ be sufficient. The following is probably an improvement:
 
   json_format($data) =~ s{</}{<\\/}rg =~ s/<!--/<\\u0021--/rg;
 
+This function generates invalid JSON if you pass it a string with invalid
+Unicode characters; I don't see how you'd ever accidentally end up with such a
+string, anyway.
+
 The following C<%options> are supported:
 
 =over