Add ltree, lquery and ltxtquery support

[halfmatthalfcat]

This adds Postgres ltree, lquery and ltxtquery support to rust-postgres. For the uninitiated, ltree ("label tree") is a Postgres datatype that follows a dot-notated search path for a given row. Lquery and ltxtquery allow users to search for rows that match an ltree. There are many query operators to help traverse the tree in a given query.

There is some commentary in #389 on why we are not adding Inners for the three datatypes. In summary, because ltree is an extension (one that ships with Postgres), it's Oids are not static like many other datatypes. This renders the Inner loading by Oid unusable and thus we must match on the Type name, like we do for citext.

Long term motivations for this are to incorporate these changes into quaint and then eventually into Prisma.

Fixes #389

[halfmatthalfcat]

@sfackler appreciate a review if you have time, open to any changes.

[halfmatthalfcat]

@sfackler I think everything should be sorted. Added more tests around the protocol parsers. Let me know if you see anything else glaring. Thanks!

[brassel]

Looking forward to see this!
Thank you @halfmatthalfcat and @sfackler!

[halfmatthalfcat]

Hey @sfackler I think since I haven't contributed before I need some assistance kicking off another build. Thanks for your help!

[halfmatthalfcat]

Ok @sfackler, sorry about that delay. I didn't know about the matches! macro and added ltree as an extension in the sql setup file, was running into a race condition during the tests.

Confirmed the latest build is working on my fork: https://github.com/halfmatthalfcat/rust-postgres/actions/runs/1996247393

I rebased as well so we should be ready to go on this.

[halfmatthalfcat]

Hey @sfackler can we kick the tires on this one more time?

[sfackler]

Kicked - sorry for the delay. I really with github would auto-build once you've been approved a single time or something... :(