add Data.Bool.ifThenElse

[Henning Thielemann]

If I activate RebindableSyntax in GHC-7.0, I have to provide a function 
named ifThenElse. How about a default implementation in Data.Bool that is 
equivalent to classical if-then-else? So far, I only use RebindableSyntax 
for overloaded number literals.