Add high level async protocol interface

[nyonson]

Add the AsyncProtocol wrapper interface for easy protocol setup, sending, and receiving.

• Feature flag gated with the new async flag.
• Example usage in the proxy crate. Makes the proxy code super simple.
• We are using futures-rs for the general AsyncRead and AsyncWrite traits to code the wrapper against. The Only annoying thing at the moment is the necessary "compat" util to convert Tokio futures to futures-rs futures in proxy, but I am not sure if there is a better way with all the different future traits.

Tests

• Tested the proxy update with Nakamoto.

Minor Cleanup

• Moved the split_garbage function into the Handshake so that it can just implicitly use the remote terminator. Also makes more organizational sense to me.
• Using default implementations for impl std::error::Error for Error {} since we were essentially duplicating the default implementation of source().

[rustaceanrob]

I think these all fall under the first commit if you could squash. Otherwise LGTM

And good find for the extension traits

[rustaceanrob]

ACK dffeef5