Consider removing reserved bytes from the first compact share

[rootulp]

Context

Since the location of the first unit (e.g. transaction) in a share is known for the first compact share in a sequence, the reserved bytes are predictable: they always contain the index immediately after the prefix. Given this observation, @cmwaters shared a great idea: it is possible to remove reserved bytes for the first compact share in a sequence. This implies that a compact share either has a sequence length (if first share) or reserved bytes (if continuation share) but never both. Given we are adopting option D in ADR 12 this has the benefit that the prefix always occupies the same number of bytes (13).

Proposal

First compact share

Continuation compact share

Consequences

Pros

• four fewer bytes of wasted space in every compact share sequence. Currently transactions are the only compact share sequence in use so this results in the marginal benefit of four fewer bytes of wasted space per block.