* Implement Debug for some structures
`NetworkConfiguration`, `TransportConfig`, `NodeKeyConfig` and `Secret`.
Needs a new release of the `rust-libp2p` crate. This PR is just a
reminder.
* Explicitly separate `std` and `core`
* Add manual implementation for Secret
Although simple, the current `Maybe*` trait implementations are based on
a manual approach that uses some code duplication. Now this duplication
is removed using a declarative macro.
Tested locally, the modification don't seem to affect building times.