Huge api change!

Everything changed, hopefully for the best.

* everything is accessible from the `na` module. It re-export
  everything and provides free functions (i-e: na::dot(a, b) instead of
  a.dot(b)) for most functionalities.
* matrix/vector adaptors (Rotmat, Transform) are replaced by plain
  types: Rot{2, 3, 4} for rotation matrices and Iso{2, 3, 4} for
  isometries (rotation + translation).  This old adaptors system was to
  hard to understand and to document.
* each file related to data structures moved to the `structs` folder.
  This makes the doc a lot more readable and make people prefer the
  `na` module instead of individual small modules.
* Because `na` exists now, the modules `structs::vec` and
  `structs::mat` dont re-export anything now.

As a side effect, this makes the documentation more readable.