Backends: abstraction of SIMD instructions

[IkerGalardi]

Right now almost all the code is similar in all backends. This duplication of code only makes development and testing harder, as fixing common issues becomes a nightmare.

To solve this a backend should be a representation of a vector with gather, load, store functions implemented. Then, a common implementation for all the backends could be implemented using the before mentioned backend (could be a class or a collection of functions.