Add iterators on sizes to Slap.Size

Iterators on integers are renamed:

- Slap.Size.fold_left -> Slap.Size.fold_lefti
- Slap.Size.fold_right -> Slap.Size.fold_righti
- Slap.Size.iter -> Slap.Size.iteri
- Slap.Size.riter -> Slap.Size.riteri

Iterators on sizes are added:

- Slap.Size.fold_left
- Slap.Size.fold_right
- Slap.Size.iter
- Slap.Size.riter

Each iterator takes a function that accepts a Slap.Size.SIZE module instead of an integer.
Improve documentation of Slap.Size