`Block::serialize_hashable` doesn't give the required blob to get the proof of work hash
Monero calculates the block ID and POW hash using slightly different blobs, for block IDs Monero appends a VarInt of the blobs length before the blob but doesn't do this for the POW hash.

I think the serialize_hashable function should return the format required for the POW hash and the id function should append the VarInt, so this should be moved out of the serialize_hashable function and into the id function:

https://github.com/monero-rs/monero-rs/blob/08d5e133cd750e5463de5c4e109c363c4985250f/src/blockdata/block.rs#L118-L120