Multiaddr::iter
* Use `unsigned-varint` crate. * Implement `Display` for `Protocol`. Gives `ToString` for free. * Use `Cow` in `AddrComponent`. * Add `AddrComponent::acquire`. * Document `AddrComponent::acquire`.
* Remove unused circular-buffer crate * Move transports into subdirectory * Move misc into subdirectory * Move stores into subdirectory * Move multiplexers * Move protocols * Move libp2p top layer * Fix Test: skip doctest if secio isn't enabled