New crate: RC6

[lqf96]

We can probably add an implementation of the RC6 cipher. Coincidentally their patents expire today (11/28/2019; source, source).

EDIT: My bad... RC6 is actually block cipher but I think it's nice to have an implementation since no vulnerabilities have been discovered is the past...

[newpavlov]

I think we can close this issue as a duplicate of RustCrypto/block-ciphers#1.