Huge overhaul of bytes

* Get rid of `ByteStr` trait
* `Bytes` is not a concrete type
* Add `BlockBuf`
* Delete lots of cruft
* Performance work

src/buf/take.rs

@@ -1,5 +1,5 @@
 use buf::{Buf, MutBuf};
-use std::{cmp, io};
+use std::{cmp};
 
 #[derive(Debug)]
 pub struct Take<T> {
@@ -52,16 +52,6 @@ impl<T: Buf> Buf for Take<T> {
     }
 }
 
-impl<T: Buf> io::Read for Take<T> {
-    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
-        if !self.has_remaining() {
-            return Ok(0);
-        }
-
-        Ok(self.read_slice(buf))
-    }
-}
-
 impl<T: MutBuf> MutBuf for Take<T> {
     fn remaining(&self) -> usize {
         cmp::min(self.inner.remaining(), self.limit)