Simple benchmark for matrix multiplication, faster default matrix multiplication, slight API change

I'm sorry in advance for a very messy PR.

This PR contains:

* a simple benchmark for matrix multiplication that compares vectorz, PersistentVector and NDArray
* a lot faster default implementation of matrix multiplication (2-3x speedup over previous PersistentVector-coercing solution);
* default matrix multiplication API and corresponding tests are slightly changed — matrix multiplication now accepts only 2 matrices (arrays of dimensionality 2) and returns a matrix (reflecting discussion in google group).