B2sum: add -l/--length option

[spineki]

Summary

This pull request adds support for the -l/--length option to b2sum (hashsum).
B2sum supports variable output size if

• length is a multiple of 8 bits
• length is less or equal than 512

Related Issue

#4610

Remarks

This is my first attempt to contribute to this project, so I may have missed something. (same goes for naming conventions) !

I added a new function to allow creating a Blake2d instance with custom output size (with_output_bytes). I updated the reset function to use this new function instead of calling ::new().
Eventually, the Blake2d tuple has now a new parameter to keep track of the custom size.

I had to create a new parsing step uu_app_opt_length that behaves more or less like uu_app_opt_bits but cannot share the same code due to different text / names.

Tests

I added a simple test with the --length option enabled. I took example of the sha1 tests in the very same file.

related test: bash util/run-gnu-test.sh tests/misc/b2sum-bsd.sh (-l flags)
Unfortunately, I only have Windows for now and I couldn't run the linux tests.

[sylvestre]

not your fault but sharing for other reviewers. the test doesn't pass because of:

2023-04-11T08:23:08.2374314Z error: unexpected argument '-c' found
2023-04-11T08:23:08.2374320Z 
2023-04-11T08:23:08.2374477Z   tip: to pass '-c' as a value, use '-- -c'
2023-04-11T08:23:08.2374482Z 
2023-04-11T08:23:08.2374575Z Usage: cksum [OPTIONS] [FILE]...
2023-04-11T08:23:08.2374580Z 
2023-04-11T08:23:08.2374724Z For more information, try '--help'.
2023-04-11T08:23:08.2374833Z FAIL tests/misc/b2sum.sh (exit status: 1)

[cakebaker]

The code looks fine :) Can you please add some tests for the error cases (length too large, length not a multiple of 8), so we don't regress in the future?

[cakebaker]

Thanks, great work :)