adjust ltree implementation

This is adevens patched version of Postgres ltree module.

Original written by Teodor Sigaev (teodor@sigaev.ru) and Oleg Bartunov (oleg@sai.msu.su)

It's different in using :: instead of . as label separator. And it allows to have special characters like { } ! * in ltree labels. You can match those label in ltree queries by escaping these characters:

SELECT '!foo::{bar}::baz%'::ltree ~ '\!foo::\{bar\}::baz\%';

Unsupported features

• Full-text queries a.k.a. ltxtquery currently does not support usage of escaped characters.