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/Util.pm

@@ -0,0 +1,17 @@
+package FU::Util 0.1;
+
+use v5.36;
+use FU::XS;
+use Exporter 'import';
+
+our @EXPORT_OK = qw/json_format/;
+
+
+sub json_format($val, %opt) {
+    my $r = FU::XS::json_format($val);
+    # XXX: Does this go over the bytes? If so, not setting SvUTF8_on() in the first place would be much faster.
+    utf8::encode($r) if $opt{utf8};
+    $r
+}
+
+1;

FU/Util.pod

@@ -0,0 +1,60 @@
+=head1 NAME
+
+FU::Util - Miscellaneous utility functions that really should have been part of
+a core Perl installation but aren't for some reason because the Perl community
+doesn't believe in the concept of a "batteries included" standard library.
+</rant>
+
+=head1 SYNOPSIS
+
+    use FU::Util qw/json_format/;
+
+    my $data = json_format [1, 2, 3];
+
+=head1 DESCRIPTION
+
+=head2 JSON parsing & formatting
+
+This module comes with a custom C-based JSON parser and formatter. These
+functions conform strictly to L<RFC-8259|https://tools.ietf.org/html/rfc8259>,
+non-standard extensions are not supported and never will be.
+
+JSON booleans are decoded into C<builtin::true> and C<builtin::false>. When
+formatting, those builtin constants are the I<only> recognized boolean values -
+alternative representations such as C<JSON::PP::true> and C<JSON::PP::false>
+are not recognized and attempting to format such values will croak.
+
+I<TODO: point to benchmarks.>
+
+I<TODO: FU::JSON wrapper with somewhat-compatible JSON::{PP,XS} API>
+
+=over
+
+=item json_format($scalar, %options)
+
+Format a Perl value as JSON.
+
+With the default options, this function behaves roughly similar to:
+
+    JSON::PP->new->allow_nonref->core_bools->convert_blessed->encode($scalar);
+
+This function croaks when attempting to format a floating point C<NaN> or
+C<Inf>.
+
+Some modules escape the slash character in encoded strings to prevent a
+potential XSS vulnerability when embedding JSON inside C<< <script> ..
+</script> >> tags.  This function does I<not> do that because it might not even
+be sufficient. The following is probably an improvement:
+
+    json_format($data) =~ s{</}{<\\/}rg =~ s/<!--/<\\u0021--/rg;
+
+=back
+
+(Why the hell yet another JSON codec when CPAN is already full of them!? Well,
+L<JSON::XS> is pretty cool but isn't going to be updated to support Perl's new
+builtin booleans. L<JSON::PP> is slow and while L<Cpanel::JSON::XS> is
+perfectly adequate, its codebase is a little too messy for my taste - too many
+unnecessary features and C<#ifdef>s to support ancient perls and esoteric
+configurations. Still, if you need anything not provided by these functions,
+L<JSON::PP> and L<Cpanel::JSON::XS> are perfectly fine alternatives.
+L<JSON::SIMD> and L<Mojo::JSON> also look like good and maintained candidates.)

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;