stat refactor

[tertsdiepraam]

This is an attempt to make the codebase of stat a bit nicer to work with. It consists of a couple of changes which I hope are relatively uncontroversial.

1. Unit tests are moved to stat.rs so that we don't have to put pub in front of everything.
2. I tried to reduce the complexity generally, by removing some weird intermediate variables, etc.
3. I made the code more idiomatic Rust as opposed to C-like. For example, instead of modelling the formatting flags as a u8, I made a Flags struct with some bool fields. Also the case of no precision being provided is now modeled with None instead of -1.
4. format! calls are preferred over some custom formatting function (e.g. extend_digits).
5. OutputType now carries it's data in the original form (e.g. as u64) instead of as a preformatted string. This unifies all the formatting to print_it, instead of being scattered between print_it and do_stat.
6. There's some minor cleanup across the file: fixing up comments, removing unnecessary type annotations, etc..

I think there's more to do, but I'm going to leave it here for this PR so it won't become too big.

[jfinkels]

Nice