cksum: Uniformize length representation in BITS rather than BYTES

[RenjiSann]

Currently, we store digest length for Blake2b and Blake3 in BYTES, and in BITS for other algorithms.
This is very error prone, since there are some functions in uucore::features::checksum that accept a size argument whose interpretation as bits or bytes comes from the context provided along this argument.

First line to change is the comment here:

coreutils/src/uucore/src/lib/features/checksum/mod.rs

Lines 252 to 254 in b7d1715

	// Note: we store Blake*'s length as BYTES.
	Blake2b(usize),
	Blake3(usize),

[RenjiSann]

Want to work on this @AldanTanneo ?

[AldanTanneo]

Yes please!