Add support for fixed length strings

[jayvdb]

Closes #392

I am not advocating arrayvec::ArrayString - it looks wrong, because dbs store chars, but this crate is about bytes.

[Electron100]

I don't think I've quite understood what you were trying to achieve here. From #392 I initially thought you were going for something that would be represented as a fixed-length string through to the SQL backend (e.g. CHAR(n) in Postgres). This is just supporting ArrayStr as a usable type in models that ultimately goes through to TEXT. Which is OK, but I want to make sure I've understood your intention correctly?

[Electron100]

I'd prefer for arrayvec (or whichever we land on here) to be an optional dependency

[jayvdb]

The goal is to be using VARCHAR(x) in the schema def. So far, this is just getting the Rust side of things. Next step is Switching from arraayvec to a type that can match VARCHAR(x) of utf8. Then fix the migrations to use VARCHAR(x).