Non-allocating versions of expm and phim

[ChrisRackauckas]

It would be nice if there could be an optional argument for the Krylov vectors so that way if this is called in a loop the caches don't need to be created each time.

[mforets]

It looks like this feature has been implemented recently in OrdinaryDiffEq.

[ChrisRackauckas]

Yup it has been. We may want to refactor that part out later, but for now it's just easier to work on them for the specific purpose (exponential integrator) all together.