feat: IS_SMALL - strongly typing SubT & UVal

[theangryangel]

I’ve completely ignored IS_SMALL and the uval field. That probably needs rectifying.

[theangryangel]

Easiest solution: UVAL is moved into the SmallType enum. i.e. (not real code).

enum SmallType {
  None,
  Ssp,
  Ssg,
  Vta(VoteType),
  … LCS(LcsType)
}

struct Small {
  reqi: RequestId,
  subt: SmallType,
}