Introduce an 'ExceptionMonad' class.

Introduce an 'ExceptionMonad' class.

This monad provides variants of 'catch', 'bracket', and 'finally', so
exceptions can be handled in monads that wrap IO.  The latter two
methods need to be part of the class definition, because GHC uses
'block' and 'unblock' which are used in the definition of those two
methods for the IO monad.  A perhaps better class interface would
consist of 'gcatch', 'gblock', and 'gunblock' and let the latter two
default to 'id' like is done for non-GHC implementations of 'bracket'
and 'finally'.