refactor(peer) some _minor_ safety features

This PR cleans up the peer package, making it a tiny bit safer to use.

* All peers are created using Peerstore. It is no longer simple to instantiate a naked peer. To create naked peers, use the `mockpeer` package.

(This refactor was performed because of work #416 and a promise I made a long time ago)
> An alternative is to put the methods in the testutil package as...

Okay, but make it `NewPeerWithID` so it's clear it's construct one every time