Add strict fmap

[quchen]

Add a strict version of fmap to the standard libraries.

infixl 4 <$!>
  
(<$!>) :: Monad m => (a -> b) -> m a -> m b
f <$!> m = do x <- m
              return $! f x
{-# INLINE (<$!>) #-}

Libraries discussions:

Take 1: ​http://www.haskell.org/pipermail/libraries/2013-November/021728.html Take 3: ​http://www.haskell.org/pipermail/libraries/2014-April/022864.html

[ekmett]

No objection here.

It also passed unanimously through the libraries@ proposal process.

[Herbert Valerio Riedel <hvr@…>]

In 0148a1c416e42a7d7c9ff3624a0640963bfe0012/ghc:

Add strict ver. of (<$>): (<$!>) to Control.Monad

A strict (<$>) has been proposed numerous times. The first time
around[1] by Johan Tibell, and the last time around[2] by David
Luposchainsky. David's thread was able to avoid The Bikeshed Monster,
and his (<$!>) proposal received unanimous +1s all around.

This addresses #9099.

 [1]: http://www.haskell.org/pipermail/libraries/2013-November/021728.html
 [2]: http://www.haskell.org/pipermail/libraries/2014-April/022864.html

Authored-by: Alexander Berntsen <alexander@plaimi.net>
Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>