[Kernel] Renaming and optimisations

- Renaming [round_to] as [change_format] to avoid confusion with [round]
- Renaming [of_float] as [represents ~float ~in_format] to better convey
  the goal of this module and why we hide the float type
- Adding [@@noalloc] to external functions [set_rounding_mode] and
  [get_rounding_mode] as a (rather small) optimisation