Field accessors/updaters

[rolyp]

Data type constructors should automatically introduce projections for extracting field values, as well as some kind of field update notation. This feature potentially interacts with #48 because fields of different constructors may have the same name - in Haskell this would be an error, but we might want to treat the duplicate field like an overloaded function/implicit typeclass. Ideally such accessors/updaters would be first-class.

• formalise as part of implementation language
• generate as part of datatype
• evaluation rules
• slicing rules

[rolyp]

Closed as no longer relevant