Implement Type-Length-Value format

[ariard]

Now it's merged in 1.1, https://github.com/lightningnetwork/lightning-rfc/blob/master/01-messaging.md#type-length-value-format (IMO we just need encoding/decoding trait for these ones, have a look in sr/util/ser.rs)

[TheBlueMatt]

Note, probably need a framework for these things but may be easier to implement after there are actual uses of them.

[dr-bonez]

https://crates.io/crates/lightning-wire-msgs

[TheBlueMatt]

Closed by #434.