Natural sort order extension.

Hi,

I've implemented a PostgreSQL extension for natural sort order. I.e.
strings like "z20", "z0004", "z11", "z2" sorted in ascending order as
"z2", "z0004", "z11", "z20".

Currently it implements the type textnso which is binary-coercible
to/from the text type. It's possible to declare table columns of type
textnso and create indexes on them.

Details are here https://github.com/dmitigr/pgnso

Any feedback are welcome!

Thanks.