More flexible Model with StructOfConstraints

[blegat]

This refactors the code of AbstractModel to make it more versatile.
Instead of having one field for each function type which contains a struct with one field for each set type, it now has a field constraints that contains one field for each function type, ...
As discussed in #1261, by changing the type of the constraints field, we can now target many more types of models.

The base is set to #1245 so that it shows the diff with that PR but this is going to be merged to master once #1245 is merged.

• Add an option to hardcode the type of the fields in StructOfConstraints instead of adding them as type parameter as otherwise the type can be huge. This option will be used by @model.
• If there is only one function or only one set, drop the corresponding layer of StructOfConstraint
• Docs
• Tests

[odow]

Do we have some benchmarks for obvious performance regressions?

[odow]

I ran the benchmarks. They're slightly slower, but I can tidy up after you've finished making large changes. It's probably just the precompile statements that need tweaking.