Huge overhaul of bytes

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

test/test_seq.rs

@@ -0,0 +1,32 @@
+use bytes::{Buf, Bytes};
+use super::gen_bytes;
+
+#[test]
+pub fn test_slice_round_trip() {
+    let mut dst = vec![];
+    let src = gen_bytes(2000);
+
+    let s = Bytes::from(&src);
+    assert_eq!(2000, s.len());
+
+    s.buf().copy_to(&mut dst);
+    assert_eq!(dst, src);
+}
+
+#[test]
+pub fn test_index() {
+    let src = gen_bytes(2000);
+
+    let s = Bytes::from(&src);
+
+    for i in 0..2000 {
+        assert_eq!(src[i], s[i]);
+    }
+}
+
+#[test]
+#[should_panic]
+pub fn test_index_out_of_range() {
+    let s = Bytes::from(&gen_bytes(2000));
+    let _ = s[2001];
+}