Add fast-forward version value to operations

[dr-orlovsky]

This should enable future fast-forward updates.

Currently version must be enforced at the type level, i.e. like

struct Transition<const VERSION: u8 = 0> {
   version: Ffv,
}

impl<const VERSION: u8> StrictDecode for Transition {
  fn strict_check(&self) -> Result<(), DecodeError> {
    if self.version != Ffv::V0 {
      return ....
    }
  }
}

[dr-orlovsky]

Closed with #151