Add impl_fmt_traits macro

[tcharding]

Add a macro that implements the fmt::{LowerHex, UpperHex, Display, Debug} traits. Includes support for displaying backwards.

Context

In bitcoin_hashes we have two versions of this code, moving it here is better because it is general hex related code. Note however that the reason for supporting display backwards might see strange unusual to the casual reader.

Note

Note please the use of naked core::foo in the macro requiring core to be in scope, is this reasonable of should we do the ugly re-export stuff we do in bitcoin_hashes: $crate::_export::_core::foo?

[tcharding]

PR is tested in rust-bitcoin/rust-bitcoin#2770

[apoelstra]

In a06b7ed:

Concept ACK. But

• We probably should do the ugly core stuff, since it guards against the case where a user has their own core module in scope.
• We should change the interface to look more Rust-like, e.g.

impl_fmt_traits! {
    #[reverse(true)] // or whatever
    impl<T: Whatever> all_fmt_traits for MyType<T> {
        const LENGTH: usize = 32;
    }
}

[tcharding]

woops, macro layout stuff sill to come. All review suggestions implented.

[apoelstra]

ACK cb9b86a I am a tiny bit tempted to say that the const LENGTH should wind up as an actual associated constant. Though this is easy to add in a followup PR

[Kixunil]

@apoelstra that's impossible because you can't write [u8; Self::LENGTH * 2] and declaring the constant anyway is weird since none of the traits have it.