Feature request: Literanger backend option

[stephematician]

I might have time to do this myself, but this looks like an opportunity to offer an alternative backed in literanger.

I've already added literanger as a backend to mice. The main reason for doing so was that it was roughly 2x as fast (in testing). I expect the gains to be more modest with missForest, but at least 1.3x should be on the cards, maybe more. It will be much faster if the prediction type is non-bagged (which makes sense with mice, but is clearly not how missForest is defined).

The interface is a little different to ranger, but overall it should have all the features needed.

[edit] fixed link to the mice PR [/edit]

[stekhoven]

Yeah, I have a couple of knits and crannies to smooth out - struggling with the authors issue in DESCRIPTION -..., might add that while I roll.