API consistency for geometric types

Changes:
* Self instead of named type
* IDENTITY, FLIP_X, FLIP_Y, FLIP_Z now constants
* Basis: make modifying methods like invert(), orthonormalize(), rotate() private
  * other APIs (Transform, Transform2D, etc) as well as GDScript ones don't have it
  * there is a naming conflict with scale() -- modifier vs. getter
  * if needed, can be re-introduced, but we have also been able to live with tons of 'self' copies so far
* Basis: rename to_scale() -> scale() in consistency with other APIs + bindings
* Basis: hange parameters self -> &self (except operators)
* Group some methods according to functionality; sys() and from_sys() at end of impl
* Remove dead code

Author: Bromeon