Use ubyte instead of humansize

[vrmiguel]

Uses Sergio Benitez's ubyte crate instead of humansize.

The main reasoning for this is removing the use of humansize's inefficient format_size function. It could be replaced by SizeFormatter::new(bytes, DECIMAL); but that seems too verbose. ubyte's usage instead looks like size.bytes() and does not allocate

As another very minor win we get to stop compiling Rust's libm

P.S. I'm not sure why we stopped using our own Bytes struct