For some reason, MSVC seems to want to instantiate these, even though
nobody ever asks for them (as evidenced by the fact that the builds
passed on other architectures).
This commit is huge, but hopefully the cognitive load is not too bad.
The bulk of this commit is just some fairly mechanical updates from
`ratio` to `Magnitude`. Other things to call out:
- `UnitRatio` goes away. We don't need this concept, because Magnitude
can't even _represent_ anything that doesn't satisfy it.
- I commented out some formatting test cases where the precise
expression changes, but the number is completely equivalent. We will
need to decide how we want to handle Magnitude formatting as a
separate, follow-on task. But at least Magnitude gives us all the
tools we'll need to do so!
Chip Hogg
Mateusz Pusz