Remove ByteOrder generic methods from Buf and BufMut (#187)

* make Buf and BufMut usable as trait objects

- All the `get_*` and `put_*` methods that take `T: ByteOrder` have
  a `where Self: Sized` bound added, so that they are only usable from
  sized types. It was impossible to make `Buf` or `BufMut` into trait
  objects before, so this change doesn't break anyone.
- Add `get_n_be`/`get_n_le`/`put_n_be`/`put_n_le` methods that can be
  used on trait objects.
- Deprecate the export of `ByteOrder` and methods generic on it.

* remove deprecated ByteOrder methods

Removes the `_be` suffix from all methods, implying that the default
people should use is network endian.

tests/test_buf_mut.rs

@@ -41,11 +41,11 @@ fn test_put_u8() {
 #[test]
 fn test_put_u16() {
     let mut buf = Vec::with_capacity(8);
-    buf.put_u16::<byteorder::BigEndian>(8532);
+    buf.put_u16(8532);
     assert_eq!(b"\x21\x54", &buf[..]);
 
     buf.clear();
-    buf.put_u16::<byteorder::LittleEndian>(8532);
+    buf.put_u16_le(8532);
     assert_eq!(b"\x54\x21", &buf[..]);
 }