Add ifThenElse and (?) to Data.Bool

[Henning Thielemann]

On Mon, 21 Nov 2016, Joachim Breitner wrote:

> It would also make teaching nicer, by pointing students to this
> function and saying „if then else is just syntactic sugar for it“.
> Slightly better than „if then else is just syntactic sugar for a
> hypothetical function that you can define, but that is not there.“

One implementation is there:
    http://hackage.haskell.org/package/utility-ht-0.0.12/docs/Data-Bool-HT.html#v:ifThenElse

if you want to point the students somewhere. :-)