Field names for value constructors

[dhess]

We would like support for (probably optional) value constructor field/parameter names. The main benefit of this feature in Primer would be for descriptive, helpful parameter names when pattern-matching against such constructors.

[georgefst]

When we do this, we'll want to add a Maybe Field field to the ConField flavor (added in #1176). Note that this will considerably change EdgeFlavors OpenAPI/JSON instances, since the type will no longer be an enum.

[georgefst]

The main benefit of this feature in Primer would be for descriptive, helpful parameter names when pattern-matching against such constructors.

This would certainly be useful, but the main benefit as far as I'm concerned would be being able to have labels in a UI describing which field is which, both when constructing and deconstructing. Consider for example the use of colours in #1173. The fact that we have no indication of which child is "red", "green" or "blue" seriously impacts the readability of programs.