RFC: Use LAPACK to estimate 1 and Inf norm. Change norm to return Float.

Inspired by the discussion of ```cond```, I have added a wrapper for the LAPACK subroutine for 1 and Inf norms. It is faster. Most of the code is now in linalg_dense.jl as it is only valid for ```StridedArray```. The norm functions now return floats for all input types.