♻️ Refactor trait into a struct

[mariocao]

♻️ Refactor

Modify the design of the implementation to use a struct with 2 generics instead of a trait.

Motivation

The trait grew to be huge and it makes more sense to have a struct with the functions use to perform a VRF.

Implementation

Some notes from @gluax :

• The trait contains all default methods and then no impl block... or proc macro
• The methods themselves should constrain the bounds
• The structs would turn into pub type P256Sha256 = EcVrf<..., ...>

Additionally, I think that we could get rid of the constants values used in the trait (e.g., PT_LEN, Q_LEN, C_LEN, ...)

[mariocao]

Already done!