Gather factors and add labels for scaled units

[chiphogg]

First, when multiplying a ScaledUnit by another magnitude, we now fold
it into the existing magnitude of the scaled unit. Previously, we'd end
up with ScaledUnit<ScaledUnit<U, M1>, M2>, and so on. We also now
omit any "trivial" scaling factors, whether because we're scaling by
mag<1>(), or (more commonly) whether we've applied a bunch of
different scale factors and they all cancel out.

(We did need to tweak a few cases that were relying on U{} * mag<1>()
being meaningfully different from U{}.)

Next, we now auto-generate labels for ScaledUnit specializations. For
ScaledUnit<U, M>, if U has label "U", and M has label "M", we
generate a label "[M U]" --- or, if "M" contains an exposed slash
"/", we'll generate the label "[(M) U]" for lack of ambiguity. This
resolves the vast majority of [UNLABELED_UNIT] labels. The remaining
work on #85 is simply to generate labels for a wider variety of
magnitude label categories.

Finally: we formerly had no way to decide ordering between units that
are both specializations of ScaledUnit, which do have identical
dimension and magnitude, and yet are not the same unit. (For
example, something like "[(1 / 4) ft]" and "[3 in]".) This may have
been somewhat obscure in the past, but with the upcoming work on #105,
it's about to become very common. We added a test case that exposes
this, and then updated our ordering code to handle this case.

Helps #85. Unblocks #105.

[geoffviola]

Looks cleaner. Thanks for the changes.