[project @ 2005-11-29 14:31:59 by ross]

As foreshadowed on the libraries list, introduce new classes:

Applicative (formerly known as Idiom): generalizes (but does not replace)
both Monad and Monoid.

Traversable: containers that can be traversed, executing actions and
re-assembling the results.  This class generalizes and replaces FunctorM,
because it requires Applicative instead of Monad.

Foldable: containers that can be folded over a Monoid.  Traversable
supplies a default definition, but some structures, e.g. Set, are Foldable
but not Traversable.