- Rename BlockedOnDeadMVar   -> BlockedIndefinitelyOnMVar
 - Rename BlockedIndefinitely -> BlockedIndefinitelyOnSTM
 - instance Show BlockedIndefinitelyOnMVar is now
     "blocked indefinitely in an MVar operation"
 - instance Show BlockedIndefinitelyOnSTM is now
     "blocked indefinitely in an STM transaction"

clients using Control.OldException will be unaffected (the new
exceptions are mapped to the old names).  However, for base4-compat
we'll need to make a version of catch/try that does a similar
mapping.