Code structure

[scrabsha]

Below is a (translated) list of suggestion @cbonaudo made on internal channels:

• rename cli.rs and glue.rs to make more explicit the functionalities they expose,
• put errors defined in glue.rs in a submodule,
• find a better name for comparaison_ref,
• put the ApiCompatibilityDiagnostics in its own module, perhaps somewhere near Comparator,
• change ApiCompatibilityDiagnostics so that it displays something when no change is detected (it will also allow us to simplify the run function).