Sort out comparisons and sorting

[AltGr]

As pointed for example in #917, we may need to clarify and implement ways of comparing elements.

Current situation

• (In)equality is polymorphic, automatically expanded to recurse into structures and enums (but that code is quick&dirty, terrible on enums)
  • Comparing functions always returns true for equality (because that is most convenient in tests and assertions)
• Comparisons are overloaded, and defined only for the base types
  • Comparison on durations can raise exceptions

Options and ideas

• Just provide a list sort primitive with syntax,that takes the comparison operation as parameter
  (might be verbose)
• Define a "canonical order" on everything, enabling comparison to be structurally expanded, like equality
  • Could be a lexical order on structs (by order of definition ? Alphabetical ?) and enums
• Define the ordering using some attributes, or extra syntax in the type definition ? (e.g. #[sort.key = 1], #[sort.ignore] ?)
• Define comparison functions manually, with an attribute expliciting that they are to be used for comparing the selected type
• Other ideas ?