Add Support for Vectored Buffers

[InsertCreativityHere]

Currently, the Encoder and Decoder only work over fully contiguous memory regions.

This is because we only implement InputSource on &[u8]
and we only implement OutputTarget on &mut [u8] and &mut Vec<u8>.
All of these types use contiguous memory.

Even if it's of limited value without memory pooling, it would be good to add implement these traits for types that utilize vectored buffers like IoSlice and Buf.