promote_eltype for LinearAlgebra and SparseArrays types #397

Would it make sense to implement promote_eltype in this package for the LinearAlgebra and SparseArrays types. I see many functions of the ArrayInterface API are already implemented for such types.
This would be helpful for MutableArithmetics which is currently trying to do it but is only doing it for Symmetric, Transpose, Adjoint and sparse arrays. See jump-dev/MutableArithmetics.jl#205