Fix/improve matrix-vector At_mul_B and Ac_mul_B

- Add ! to end of matrix-vector At_mul_B that takes output array
- Use BLAS for Ac_mul_B! with BLAS types

It doesn't look like there are presently any tests for matrix-vector `A_mul_B` etc. or for BLAS in-place multiplication methods. (The only in-place tests in `linalg.jl` test generic multiplication with Ints.)