Add async send and recv functions to AsyncSession

[AaronKutch]

If I am understanding this correctly, this is equivalent to the strategy already used in the AsyncRead and AsyncWrite implementations. These functions take &self which is needed for sanity downstream.

The only thing is that I am concerned about the writing side which has no polling mechanism or anything. Would it become effectively blocking with high packet rates?

[AaronKutch]

I have tested it with a modified rust-tun (which requires nothing more than replacing the two unimplemented! with self.session_reader.session.recv(buf).await and self.session_reader.session.send(buf).await, and it appears to work with decent UDP throughput. TCP is having a hard time initiating though, I'm not sure if it's because of one of my layers.
This is needed to fix ssrlive/rust-tun#109.

[ssrlive]

I have pushed a patch for it. Please report your testing result here.

[AaronKutch]

It is working much better now. Thank you!