Parallel matrix multiplication is broken.

[eholk]

It's currently commented out in the benchmarks so the tests keep passing. Here's the error we get:

./build/main
Benchmarking 1200u x 1200u matrices.
Matrix Multiply: 7.01859f64s
Matrix Multiply (blocked): 15.500061f64s
(1200u, 1200u)
task '<main>' failed at 'assertion failed: lhs.num_cols() == rhs.num_rows()', src/SciRust/matrix/par.rs:25

Since the matrices are the correct size when we call mat_mul, but the assertion immediately fails, I has some suspicions this might be a bug in Rust. That's why I didn't fix it for the 0.2/rust-0.10 release.