Reimplement fmt::Debug for Bytes and BytesMut (#84)

Standard `Debug` implementation for `[u8]` is comma separated list
of numbers. Since large amount of byte strings are in fact ASCII
strings or contain a lot of ASCII strings (e. g. HTTP), it is
convenient to print strings as ASCII when possible.

tests/test_bytes.rs

@@ -43,7 +43,7 @@ fn from_slice() {
 #[test]
 fn fmt() {
     let a = format!("{:?}", Bytes::from(&b"abcdefg"[..]));
-    let b = format!("{:?}", b"abcdefg");
+    let b = "b\"abcdefg\"";
 
     assert_eq!(a, b);