Add sort function?

$cmp = sub { ... }; # where $a/$b = [$k, $v]

$oh->sort( $comparator );

Or should users just create sorted copies with clone and
their own list of keys?

If adding a sort, should it optionally take Schwartzian
tranform arguments for efficiency/utility?

$oh->sort( $cmp, $xin, $xout );

Essentially, default $xin and $xout are these:

$xin  = sub { [ $_, $_[0]->{$_} ] };
$xout = sub { $_->[0] };

Called like this:

@keys = map { $xout->($self) } sort $cmp map { $xin->($self) };

This might be a lot more complexity and indirection vs just using clone:

$oh = $oh->clone( map { ... } sort { ... } map { ... } $oh->keys );

So maybe not allowing custom $xin and $xout is best, but do a ST internally
to [$k, $v] for a bit of flexibility in ordering.