feat(lib): switch to non-blocking (asynchronous) IO

BREAKING CHANGE: This breaks a lot of the Client and Server APIs.
  Check the documentation for how Handlers can be used for asynchronous
  events.

src/header/common/host.rs

@@ -1,4 +1,4 @@
-use header::{Header, HeaderFormat};
+use header::{Header};
 use std::fmt;
 use header::parsing::from_one_raw_str;
 
@@ -52,9 +52,7 @@ impl Header for Host {
             // https://github.com/servo/rust-url/issues/42
             let idx = {
                 let slice = &s[..];
-                let mut chars = slice.chars();
-                chars.next();
-                if chars.next().unwrap() == '[' {
+                if slice.starts_with('[') {
                     match slice.rfind(']') {
                         Some(idx) => {
                             if slice.len() > idx + 2 {
@@ -86,9 +84,7 @@ impl Header for Host {
             })
         })
     }
-}
 
-impl HeaderFormat for Host {
     fn fmt_header(&self, f: &mut fmt::Formatter) -> fmt::Result {
         match self.port {
             None | Some(80) | Some(443) => f.write_str(&self.hostname[..]),