Logging

[hadronized]

spectra needs to log things. Logging is done by using a user-provided object that logs whatever spectra needs to. A trait is currently available in spectra::logger.

Left to do:

• [x] 5fc33b2 Ensure the logger public interface is correct.
• [x] Brainstorm about where the logger should be passed (should the whole spectra ecosystem has a Context type containing it?

[hadronized]

The current code requires someone to depend on both spectra::app::runner and spectra::resource::context to build an application. Maybe we could lift that into spectra::app directly? The idea is that:

• Writing a debug / release / whatever application should be easily constructible.
• A context is an object that can be composite. That is, it might contain a logger but also something else that might be needed by another part of spectra.

The last point shows that we should accept a type as argument to runner builders and constraint them with the required traits.