[byteorder] Remove byteorder feature and crate dep

[joshlf]

Remove the byteorder feature and the dependency on the byteorder crate. Replace the ByteOrder trait and types which implement it from the byteorder crate with our own native implementations. This allows us to make some of our functions and methods const.

Add Usize and Isize byte order-aware types.

Closes #438

[joshlf]

As it stands, this won't compile on our MSRV because it doesn't support const fns with trait bounds (in this case, O: ByteOrder). If we want to keep this as-is, we'll need to land #574 or similar first so that we can have these be conditionally const.

[joshlf]

The rustdoc CI failures seem to come from the fact that, in define_type!, we use stringify! on this expr in order to generate the doc comment.

I think the simplest thing to do here might just be to introduce a new macro argument that is used in the summary sentence of the doc comment instead of doing stringify!($bits). It would have the text, "A 16-bit unsigned integer" , "A word-sized signed integer", etc. i.e.:

Old

        doc_comment! {
            concat!("A ", stringify!($bits), "-bit ", $number_kind,
            " stored in a given byte order.

New

        doc_comment! {
            concat!($description, " stored in a given byte order.

This would also allow us to get rid of the $number_kind argument.