Fix exp_generic for general AbstractMatrix.

[chriselrod]

Fixing a couple oversights and adding tests to make sure StaticArrays.MMatrix still works, as an example generic mutable AbstractArray.

Those oversights were:

1. Base.Experimental.Const is only defined for Array.
2. lu! is only defined for StridedMatrix. MMatrix doesn't subtype strided, and also does not return LinearAlgebra.LU, returning StaticArrays.LU instead. The latter doesn't support rdiv!.

While it'd be nice to fix "2." in StaticArrays, for now it's easiest to just work around it here.