Decode from `&mut Input` rather than from `&mut &[u8]` to align with scale_codec

[jsdw]

It would be nice if this library would play well with parity-scale-codec's Decode trait. this trait takes in an &mut Input (something implementing the Input trait), but because scale-decode only accepts &mut &[u8], we can't easily use it as part of that decode impl.

Let's consider supporting &mut Input instead to allow these crates to play nicely together.