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default implementations of 'gbracket' and 'gfinally' just work.

MERGE TO 6.10

Ifdefs for whether we had extensible exceptions or not were spreading
through GHC's source, and things would only have got worse for the next
2-3 years, so instead we now use an implementation of extensible
exceptions built on top of the old exception type.

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'.

TopHandler now uses the new extensible exceptions module, so we
need to interact with it using the new types.