Benefits:
* Blake3 is faster. This is most noticeable for the hashing of the
sample file data.
* we no longer need OpenSSL, which helps with shrinking the binary size
(#70). sha1 basically forced OpenSSL usage; ring deliberately doesn't
support this old algorithm, and the pure-Rust sha1 crate is painfully
slow. OpenSSL might still be a better choice than ring/rustls for TLS
but it's nice to have the option.
For the video sample entries, I decided we don't need to hash at all. I
think the id number is sufficiently stable, and it's okay---perhaps even
desirable---if an existing init segment changes for fixes like e5b83c2.
