wip/hs-fclabels 1.0.4 NetBSD

First class labels that act as bidirectional record fields.

The labels are implemented as lenses and are fully composable and can be used to get, set and modify parts of a datatype in a consistent way. The lens datatype, conveniently called `:->', is an instance of the Category type class: meaning it has a proper identity and composition. The library has support for automatically deriving labels from record selectors that start with an underscore. Labels can be used in a purely functional setting or be applied to mutable state in some state monad.