tokio-postgres-rustls

• Minimize API
• Add documentation
• Eliminate allocations
• Migrate from x509-certificate to x509-cert to avoid policy problems with x509-certificate's MPL license in some organizations

Thanks to @aumetra for the above improvements!

• Update rustls to 0.23 (thanks @aumetra!)

• Update x509-certificate to 0.23 to avoid pulling in multiple versions of ring. Thanks @serprex

• Fixed the SCRAM channel binding check to use the correct digest based on the server certificate (thanks @JamesGuthrie)
• Updated rustls to 0.22 (thanks @ol-teuto)
• Updated ring to 0.17 (thanks @fnichol)

• Updated rustls to 0.20 (thanks @UjOwtuc!)

Deferred failure in make_tls_connect (e.g. due to an invalid/missing hostname) until establishing the TLS connection is actually attempted. This is consistent with the behaviour of other tokio-postgres TLS integrations, and allows using SslMode::Disable to e.g. connect to a UNIX socket whilst still having tokio-postgres-rustls as the chosen TLS library for tokio-postgres.

• Updated to tokio 0.3, rustls 0.19, tokio-postgres 0.6, tokio-rustls 0.21.