Remove IntoBuf/FromBuf (#288)

As consequence Buf::collect is removed as well, which is replaced with `Buf::into_bytes`. The advantage of `Buf::into_bytes` is that it can be optimized in cases where converting a `T: Buf` into a `Bytes` instance is efficient.

tests/test_buf_mut.rs

@@ -32,7 +32,7 @@ fn test_vec_as_mut_buf() {
 #[test]
 fn test_put_u8() {
     let mut buf = Vec::with_capacity(8);
-    buf.put::<u8>(33);
+    buf.put_u8(33);
     assert_eq!(b"\x21", &buf[..]);
 }