Add List.foldr (++) empty to concat simplifications

[lue-bird]

List.foldr (++) "" list
--> String.concat list

List.foldr (++) [] list
--> List.concat list

Implementing the first simplification closes #317
(Also when reviewing, the initial fix contains a bug that is later fixed)

Also fixes a rare bug when reducing lambdas

[jfmengels]

Looks great, thanks!