Just some minor optimizations I figured out a while ago. By using ReadBytes instead of
Bytes on the rlp stream, we can save the allocation of a temporary buffer for the typed tx
payload.

If kind == rlp.Byte, the size reported by Stream.Kind will be zero, but we need a buffer
of size 1 for ReadBytes. Since typed txs always have to be longer than 1 byte, we can just
return an error for kind == rlp.Byte.

There is a also a small change for Log: since the first three fields of Log are the ones that 
should appear in the canon encoding, we can simply ignore the remaining fields via 
struct tag. Doing this removes an indirection through the rlpLog type.

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Co-authored-by: Martin Holst Swende <martin@swende.se>