Feature request: RC5 & RC6 Ciphers

**Is your feature request related to a problem? Please describe.**
The ciphers for RC5 and RC6 are missing

**Describe the solution you'd like**
Please consider adding support for these ciphers

**Describe alternatives you've considered**
Using other tools or creating custom one off decoders is the current workaround

**Additional context**
https://en.wikipedia.org/wiki/RC5
https://en.wikipedia.org/wiki/RC6

The issue names no files, entry points, or tests. Start by reviewing the linked RC5 and RC6 references and the project's existing cipher support; done means both ciphers are implemented with agreed variants, inputs, outputs, and tests.

Written by the indexing model from the issue text.