Imlement glm::decompose

Hi

I've been using the `glm` crate to great satisfaction last few weeks. I'm here to request that you could implement a version of the `glm::decompose` function as defined [here](https://glm.g-truc.net/0.9.6/api/a00204.html).

It is extremely useful for doing camera controls where you want to separate out translation and rotation out of a mat4.

Thanks a lot.