Improve performance in debug mode

This issue originates from a request made by @happenslol [there](https://github.com/rustsim/nalgebra/issues/217#issuecomment-440804721).

Much effort has already be made to improve performance of nalgebra when compiled with full optimization (O3 and link-time optimization enabled). Now we should investigate how to improve performances in debug mode (or with low optimization levels). Here are two possible leads:

- Lack of explicit SIMD. Adding explicit SIMD to nalgebra could yield significant performance improvements.
- Non-zero cost abstractions in debug mode. Unfortunately, abstractions are zero-cost only when optimization are enable. We should investigate in particular the occurrence and cost of:
- Nested function calls.
- Indexing (unnecessary bound checking for statically-sized matrices).
- Debug assertions?
- Loops that are not unrolled.
- Trivial checks and branching that are not removed by the compiler (for example checks like [that](https://github.com/rustsim/nalgebra/blob/master/src/base/blas.rs#L198) are automatically evaluated at compile-time and removed by the optimizing compiler for statically-sized vectors).

I am not sure yet if we can make significant improvements using the stable version of the compiler. However, we could benefit from specialization and explicit SIMD using the nigthly compiler.