Fix build errors: socket2 API, rustls trait impl, async field access,…

[fewensa]

… Alphanumeric type

• connection/mod.rs: remove incorrect #[cfg(feature = "async")] guard on block_connection
• connection/connection.rs: replace socket.into_tcp_stream() with std::net::TcpStream::from(socket)
• connection/connection.rs: implement missing ServerCertVerifier methods and #[derive(Debug)] for NoCertificateVerification
• connection/connection.rs: call .to_owned() on ServerName to fix lifetime issue
• connection/connection.rs: use Display instead of Debug for native-tls HandshakeError
• request/builder/build_body_async.rs: use self.request field directly instead of self.request() method
• request/builder/form_data.rs: cast u8 to char for Alphanumeric distribution output