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.