refactor(header): change name of deref! to __hyper__deref!

The header is largely for internal use, however, it must be exported
because the header! macro uses it. As deref! is also a particularly
general name, this renames it to __hyper__deref! to prevent name
clashes.

BREAKING CHANGE: If you use deref! from the header module, you'll need
  to switch to using __hyper__deref!.

src/header/common/cache_control.rs

@@ -23,7 +23,7 @@ use header::parsing::{from_one_comma_delimited, fmt_comma_delimited};
 #[derive(PartialEq, Clone, Debug)]
 pub struct CacheControl(pub Vec<CacheDirective>);
 
-deref!(CacheControl => Vec<CacheDirective>);
+__hyper__deref!(CacheControl => Vec<CacheDirective>);
 
 impl Header for CacheControl {
     fn header_name() -> &'static str {