Consistent number conversion function names:
* `mrb_value` to immediate (C) value
  * `mrb_int()` -> `mrb_as_int()`
  * `mrb_to_flo()` -> `mrb_as_float()`
* `mrb_value` to `mrb_value` (converted)
  * `mrb_to_int()'
  * `mrb_Integer()` - removed
  * `mrb_Float()` -> `mrb_to_float`

Consistent function name (avoid `_flo` suffix):
* `mrb_div_flo()` -> `mrb_div_float`