Deal with serde_bytes module

[fjarri]

For now it's just a fix to handle Box<[u8]> correctly in serialized messages. The problem is:

• it doesn't support other possible byte holders, e.g. stack arrays, which we may want to use in some places (like SessionId),
• it doesn't support human-readable formats

We need to find a good crate that does all that, or finally make our own.

[fjarri]

To give some background:

• There is an old open issue Support for human-readable formats? serde-rs/bytes#37, which likely won't get fixed
• See specifically this comment enumerating existing libraries, none of which implements the functionality that we want (and given their respective intents, never will)

So, as much as I wouldn't want to add another similar-but-not-quite-the-same library to the ecosystem, that seems like the only viable option.

[fjarri]

To be replaced with http://crates.io/crates/serde-encoded-bytes