Adding an ignore function to Control.Monad

[Gwern Branwen]

OK, let's apply a little democracy here. Reading back through the
thread, I tally the votes:

# Nothing
David Menendez
John Meachem (?)

# Just
## Control.Monad.ignore m a -> m ()
Michael Snoyman
Neil Mitchell
Isaac Dupree
Maurí­cio
Martijn van Steenbergen

### Control.Monad.ignore f a -> f ()
Edward Kmett

## Control.Monad.void m a -> m ()
Don Stewart
Iavor Diatchki

--
gwern