Support for Rust async/await

• All oftokio-socketcan has been merged into this crate and will be available with anasync-tokio build feature.
• #41 Added initial support forasync-io for use withasync-std andsmol
• SplitSocketOptions trait out ofSocket trait for use with async (breaking)
• Added cargo build features fortokio orasync-io.
• Also created specific build features forasync-std andsmol which just bring in theasync-io module and alias the module name toasync-std orsmol, respectively, and build examples for each.