use Blake3 instead of SHA-1 or Blake2b

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.

guide/schema.md

@@ -250,6 +250,10 @@ Version 6 adds over version 5:
 *   metadata about the pixel aspect ratio to properly support
     [anamorphic](https://en.wikipedia.org/wiki/Anamorphic_widescreen) "sub"
     streams.
+*   hashes in Blake3 rather than older SHA-1 (for file integrity checksums)
+    or Blake2b (for sessions).
+
+On upgrading to this version, sessions will be wiped.
 
 Before it is finalized, it likely will also add a schema for [object
 detection](https://en.wikipedia.org/wiki/Object_detection).