[PATCH] Expand character set for ltree labels

Dear hackers,

I am submitting a patch to expand the label requirements for ltree.

The current format is restricted to alphanumeric characters, plus _.
Unfortunately, for non-English labels, this set is insufficient. Rather
than figure out how to expand this set to include characters beyond the
ASCII limit, I have instead opted to provide users with some mechanism for
storing encoded UTF-8 characters which is widely used: punycode (
https://en.wikipedia.org/wiki/Punycode)

The punycode range of characters is the exact same set as the existing
ltree range, with the addition of a hyphen (-). Within this system, any
human language can be encoded using just A-Za-z0-9-.

On top of this, I added support for two more characters: # and ;, which are
used for HTML entities. Note that & and % have special significance in the
existing ltree logic; users would have to encode items as #20; (rather than
%20). This seems a fair compromise.

Since the encoding could make a regular slug even longer, I have also
doubled the character limit, from 256 to 512.

Please let me know if I can provide any more information or changes.

Very sincerely,
Garen