added a convert tool between bech32 and hex

[hackphobic]

Description of change

Added a conversion tool between Bech32, Hex and public key encoding.

Links to any relevant issues

https://github.com/iotaledger/bee/issues/263

Type of change

• Enhancement (a non-breaking change which adds functionality)

How the change has been tested

I included 3 unit tests for each function. The conversions are available in the spec page https://github.com/iotaledger/tips/blob/main/tips/TIP-0011/tip-0011.md

Change checklist

• [x] I have followed the contribution guidelines for this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] I have checked that new and existing unit tests pass locally with my changes
• [ ] I have updated the CHANGELOG.md, if my changes are significant enough

[hackphobic]

@thibault-martinez done! the licenses test seems to be failing on one of the imported crates.

[thibault-martinez]

@thibault-martinez done! the licenses test seems to be failing for one of the imported crate.

Yeah don't worry about that. I'll review tonight or tomorrow, thanks.