Allow access to all inner streams [tokio-native-tls]

[aloucks]

Currently the get_ref and get_mut accessors skip over the intermediate stream which prevents access to things like TlsStream::peer_certificate. This change makes the AllowStd type public and provides a way to access all layers of the nested inner streams.

Related: tokio-rs/tokio#1383

[aloucks]

The lint check is failing due to lack of cargo fmt, however when I ran it, the entire project was reformatted.

I was able to fix the compile errors for the smoke tests, but macos is still failing due to some invalid flags passed to s_server.

[LucioFranco]

@aloucks looks like there is a merge conflict, if you can get that resolved we can merge!

[aloucks]

I rebased and resolved the conflicts but there are some compile errors in rustls crate.