Adding support for citext[] arrays.

[jwoertink]

Fixes #970

This PR will add support for citext[] which is an array of case-insensitive strings. This would allow you to store an array of strings in a column, and query against it without worrying about the casing (e.g. tags)

UserQuery.new.tags.includes("aNyCasE")

Currently putting this in WIP because Crystal PG doesn't have array decoder support for citext

Ref: will/crystal-pg#250

From what I can tell, when you add in the citext extension, that postgres server will assign citext a custom OID which means the OID may be different on every project, server, etc... Since CrystalPG can't just determine what the type is when using a StringDecoder, it ends up raising an exception.

My guess is once CrystalPG supports it, most of this code will "just work"... However, one tricky part is postgres doesn't seem to auto-cast between text and citext when it comes to arrays 😝 It can do WHERE email = 'SuPpOr@blah.COM' but it can't do WHERE tags @> array['Crystal']. You have to cast the array like array['Crystal']::citext[]. So if that doesn't auto happen through crystal PG, then we may need to make sure your column definition knows it's a citext somehow.