rc6: RC6 Implementation

[ashWhiteHat]

Part of https://github.com/RustCrypto/block-ciphers/issues/1

Hi there. I implemented rc6.

Implementation and documentation are aligned with rc5 implementation.

I referred following document for key schedule, encryption and decryption algorithms. https://www.grc.com/r&d/rc6.pdf

I checked whether it works by following test vectors. https://datatracker.ietf.org/doc/html/draft-krovetz-rc6-rc5-vectors-00#section-3

I would appreciate it if you could confirm. Thank you!

[ashWhiteHat]

Ref https://github.com/RustCrypto/block-ciphers/pull/371#issuecomment-2258690541

Hi @tarcieri I rebased PR.

I would appreciate it if you could confirm. Thank you.

[Testsr]

Anything?