Refactor `NumberToRoundedConverter`

* Clarify the pieces of `NumberToRoundedConverter` with better
names and with more idiomatic Ruby.
* Begin to eliminate hidden meaning in a few of the mathematical
  operations trying to round the numbers correctly.
Eliminate duplication in options definition

Many of the Number converters are using the same pattern
to determine locale formatting options. To eliminate some
duplication, move this similarity to the `NumberBuilder` class
to allow all of the other number builders to take advantage
of it.