An internal representation of a wallet seed.
This contains 19 bytes of entropy, 17 of these bytes are pure random data
and two of them represent the seed’s “birthday”, i.e. the day when the seed
was first created.
By including a “birthday” in the seed, wallet implementations can avoid scanning
the entire history of the blockchain to look for transactions when the wallet may
have been paid. The birthday bytes are also used as entropy.