RFC: Use Number instead of Integer in some linalg operations

I need rank to work for matrices with rational entries, so I replaced Integer by Number in some places. I hope this is not too general (I think it used to be something like this in an earlier version, so maybe there was a good reason to remove it)

@ViralBShah: I believe you are also thinking about this (https://github.com/JuliaLang/julia/issues/2930), do you think this is the way to go?