v0.8.0

* Extensive rewrites throughout
* SslStream handshakes no longer immediately complete with nonblocking sockets.
  A `MidHandshakeSslStream` is returned instead which can restart the process.
* RSA-specific functionality on PKey was removed.
* Crypter has been rewritten to support no-allocation use.
* Error types have been unified
* `openssl` no longer requires OpenSSL headers by default.
* HMAC support and some other minor features are now behind feature flags and
  do require headers.