RFC: Add allocation free matrix multiplication and solving

Here are the allocation free versions of the dense matrix algebra. I have tried to make all the versions use the BLAS way of updating. It is a major rewrite of `matmul` so this should wait for after 2.0, I think. The `generic_matmatmul!` is a little fancy so I am not sure I got it right, so please review it.

This was motivated by some work on iterative methods for solving linear systems where it was useful to have a common name and API for the allocation free multiplication. I hope that find an API such that you are free to define a type and `A_mul_B!` and then you can start solving linear systems and eigensystems efficiently.