remove the dependency on byteorder

[oconnor663]

Since that crate takes slices in its API, it tends to introduce unnecessary bounds checks.

[oconnor663]

This removes a dependency and slightly improves the performance of the portable implementation. But we can't land it until rust-lang/rust#52963 is stable, unless we want to copy those unsafe functions into the crate.

[oconnor663]

Notably, with the slight performance improvement in this PR, our b2sum hardcoded to use portable.rs edges past the coreutils b2sum in performance.

[oconnor663]

Actually it turns out that the slight performance drag was entirely due to read_u64_into, and unrolling that loop improves it, without needing to remove the byteorder dependency. I'm going to land some of that now, and we can drop this PR. The to_le_bytes API will be nice, but I think at least in this case it's going to compile to the same code as byteorder.