Add execWriterT

Funny, I actually originally committed it with Apply and <$>, but I looked at existing code and saw that it used Monad so I decided to stick with convention. Given this information, should I go with Apply, and additionally would you like the following code updated to use Apply as well?

https://github.com/purescript/purescript-transformers/blob/master/src/Control/Monad/State/Trans.purs#L16