Use of `bitcoin` in `protocol`

[rustaceanrob]

In working on #149, I found that the only dependency on bitcoin, and p2p for that matter, is for Magic from Network. I think we should consider removing bitcoin entirely. My guess is any application that gets the initial magic wrong will be in early development or have a very critical bug related to their storage of what Network they are on. The loss of the serde module is acceptable with the current state of p2p.

Process to complete this would be something like:

• Remove explicit rand dependency #151
• Update secp256k1 to hashes 0.16.0 Update hashes to 0.16.0 rust-secp256k1#832
• Update to the 0.1.0 of p2p
• Remove the serde module for p2p

[apoelstra]

Sounds like we should move Network into its own crate ASAP, and then you could just have a dependency on that.

Or do you think that even that's unnecessary?

[rustaceanrob]

The only use of Network here is to derive Magic, in which case we would likely just use an extension trait for that new Network to get the Magic internally. I believe the better approach is to use the no_std version of p2p directly with no notion of a network at all, and thus no bitcoin and Network crate.

For this crate in particular, I also think a four byte slice is acceptable, as they will never establish a connection on their desired network if its wrong. Also hacky but interesting, just using 4 byte slices would enable uses that are not tied to a network at all (maybe people want to exchange PSBTs or something)

[apoelstra]

Makes sense. Sounds good to me!

[nyonson]

Should we just drop the dep on p2p too? We can keep an example of how to use it in the dev-deps, but isn't really necessary for the protocol crate and might simplify our crate tree in the higher up p2p libs we are workin on.

[rustaceanrob]

On the public facing API, we could type alias [u8; 4] as Magic and call it a day, but I think the problem is more in the development dependencies. If we gutted bitcoin right now, I think traffic and proxy, not to mention the tests, all depend on either the serde module or p2p to facilitate the encoding. Removing bitcoin right now seems to cause more of a headache than an advantage, so I figured we could wait for the next release cycle to decide.