Primitives for operating with Yubikeys (mostly around OTP parts).

Assumptions:

• most stuff like keys are encoded in hex, to be compatible with the reference implementation
• structs have GORM/JSON/YAML annotations where appropriate so they can be used directly in GORM and/or API
• functions operating on modhex/hex just replace invalid characters with X. Normally that is either checked or just makes hex module fail on conversion so it is safe and fast to use inside the module but take care, giving wrong output to ModHex2Hex for example will return invalid hex string