Update `FixedPointNumber` and other Arithmetic with Proper Names

[shawntabrizi]

#8327 introduced a better naming for functions across PerThing.

The same naming should be applied to all the other helpers and utilities within Substrate Arithmetic.

cc @kianenigma

[kianenigma]

I checked the names now and this is mostly done, there is just one function name that needed fixing. I'll do this as a part of another PR.