Keep in mind that this is probably not the best tool for this task,
because it is written in a slow(er) language, as well as it ignoring the
advice for searching for vanity addresses given in the specification.
Usage
$ curvy --prefix="foo" --output="./hs_ed25519_secret_key" --threads=64
public key: foob64oywfdxkr4mxxio4xis6lba2peoplvj64kqbpybd2n6sqq6joqd.onion
TODO
Rewrite this in a language that natively supports threads and erases secret keys securely