Fix mat4::from_quat
Add << overload for mat4. Debatable if I like this.
Fix methods used in mat4's operator *.
Add inver(se|t) to vec[34]. Fix operator- on quat.

* Add unary ~ for the inverse of quaternion and vector types.
Add operators for vec3/4 and mat4.

* / and /= for vec3 and vec4 to scale by the inverse of vectors.
* - and -= for mat4 to get the difference of two matrices.