Try out scroll or zerocopy for de/encoding of PDUs

[jonas-schievink]

scroll allows deriving decoding and encoding traits, which is just what I want. We should try porting rubble to use that instead of bytes.rs.

We can also try out the zerocopy crate, which allows C-like reinterpretation of arbitrary bytes in safe Rust. Unfortunately it's near impossible to contribute to zerocopy since it's part of fuchsia, but it looks promising.

[jonas-schievink]

It'd be pretty easy to add a method to ByteReader that reads any type implementing zerocopy::FromBytes. Then we can gradually move hand-written deserialization code to make use of that.