Introduce checked `PerThing` arithmetics

[ggwpez]

Some PerThing functions are prone to zero division panics. This requires the runtime code to pay close attention when calling them.
Instead of relying on proof comments we could add fallible checked functions for:

• multiply_by_rational_with_rounding
• rational_mul_correction

(there could be more, did not check all)

Context: paritytech/substrate#13660 (comment)