Consider TypeState in pipeline/Sector

[th7nder]

          Nice. new typestate pattern makes it more readable. You can also use generics if you don't want to copy shared state like `sector_number`, `deals`, `piece_infos` between structs.

Something like:

struct Sector<State> {
    <shared>
    state: State
}

struct Unsealed {
    <unsealed specifics>
}

struct PreCommitted {
    <precommitted specifics>
}

struct Proven {
    <proven specifics>
}

Originally posted by @cernicc in #502 (comment)

[jmg-duarte]

Closing since this is something that we'll figure out as we build