Maybe refactor `shredder` module

[qkniep]

Maybe something like this

  src/shred.rs                          Shred, ShredPayload, ShredPayloadType, wire consts
  src/shred/
  ├── validated.rs                      ValidatedShred, ShredVerifyError
  └── index.rs                          ShredIndex

  src/consensus/shredder.rs             Shredder trait, ShredError, DeshredError
  src/consensus/shredder/
  ├── reed_solomon.rs
  ├── validated_shreds.rs               pub(crate), algorithm-internal
  ├── pool.rs                           ShredderPool, ShredderGuard
  ├── regular.rs                        RegularShredder
  ├── coding_only.rs                    CodingOnlyShredder
  ├── pets.rs                           PetsShredder
  └── aont.rs                           AontShredder