b3sum: digest checksum for multiple files

[daviessm]

I have a project with a requirement to output a hash for each file and an overall hash of the whole set of files at the end. This could be achieved with b3sum file1 file2 file3 > filesums; b3sum filesums but a built-in option would be nice too.

I started work on this in https://github.com/daviessm/BLAKE3/commit/d6be56658ec8c0f3075bb3254bc248026aa38a60 but as a Rust newbie I don't know if I'm going down the right path with this.