Use fixed-sized types in redb to avoid storing lengths

[casey]

I think if we use fixed-size types, we'll save a u64 or u32 length. @cberner is this right?

So we can change our table definitions to:

const HEIGHT_TO_HASH: TableDefinition<u64, [u8; 32]> = TableDefinition::new("HEIGHT_TO_HASH");

const OUTPOINT_TO_ORDINAL_RANGES: TableDefinition<[u8; 36], [u8]> =
  TableDefinition::new("OUTPOINT_TO_ORDINAL_RANGES");

const OUTPOINT_TO_TXID: TableDefinition<[u8; 36], [u8; 32]> = TableDefinition::new("OUTPOINT_TO_TXID");

[cberner]

Yep! You'll save a u32 length for every key and/or value