Add vector casts as needed.
Change typedef for string to using.
Add rotation/multiply for additional vectors.
Add functions for iterating over objects in pools.
Add slot_* types.

These are probably relatively slow types to use but handy for image
packing and pre-processing data. They barely track what slots are
consumed/released except to give them handles, so you will have to keep
slot position/dimensions/handle around if you plan to consume and
release slots as needed.