Remove some useless #[test] tags.
Add a `new_random` method to build a `DMat` or `DVec` filled with random numbers.
Add unsafe methods to initialize a DVec or DMat without initialization.
Add an unsafe `at_fast` method for unchecked read access to DVec and DMat.
More unsafe indexing in inner loops and add some benchmarks.
Specialized Mul, RMul, and LMul for Mat2 and Mat3.