forward specialized BytesMut methods on {Encode,Decode}Buf

[srijs]

Forwards more Buf/BufMut methods on EncodeBuf and DecodeBuf to take advantage of specialized implementations on BytesMut, where available.

Motivation

BytesMut has some optimized variants of Buf and BufMut methods that users of EncodeBuf and DecodeBuf currently miss out on. In particular, an O(1) zero-copy variant of Buf::copy_to_bytes and variants of BufMut::put and BufMut::put_slice that elide checking remaining_mut/advance_mut.

Solution

Provide specialized implementations of Buf::copy_to_bytes on DecodeBuf as well as ButMut::put and BufMut::put_slice on EncodeBuf which forward these calls onto the underlying BytesMut implementations.

[tottoto]

Thanks. Seems to be addressed at #1695.