Indicate relative magnitudes in common unit labels

[chiphogg]

A core part of Au's philosophy is "no preferred units". So when we subtract, say, MPH from KPH, the result is labeled as essentially "the common unit of MPH and KPH": COM[km / h, mi / h].

I think this is a mixed bag. It's good that we don't conjure up any unmentioned units. COM[km / h, mi / h] does unambiguously identify what unit is being used. But this isn't a commonly used unit, and it's clearly bad that the label doesn't give any clue as to its magnitude, relative to any other unit.

I've found a new candidate solution. We can express the common unit in terms of each constituent unit --- and, for clarity, replace the , with ==. For example: COM([1 / 15625] km / h == [1 / 25146] mi / h).

• PRO: Doesn't put any unit on special footing.
• PRO: Clearly indicates the common unit's magnitude in terms of each constituent unit.
• CON: A little verbose.

I think this is a clear improvement. Common-unit quantities generally only occur as the result of intermediate computations. They're not the kind of thing that end users are going to want to print, unmodified. But when they do end up being output, they'd better be unambiguous!

This solution depends on #85, because we need to be able to print magnitudes.

[chiphogg]

Related: mpusz/mp-units#438.