* PBKDF2-HMAC-SHA256 and -SHA512 functions
* SNI support in SslStream
* A major rewrite of SslStream internals
* read and write are now invoked directly on the wrapped stream with no
intermediary buffering via a custom BIO.
* NonblockingSslStream is deprecated in favor of SslStream::{ssl_read,
ssl_write}
* connect_generic and accept_generic are now equivalent to connect and
accept
* A new error type, ssl::Error, is returned from ssl_read and ssl_write,
and will be the new return type of all other methods in the 0.8 release