Treat combined durations as a single unit

Prior to this commit, `3.months - 3.months` would result in a duration
that has the "parts" of `[[:months, 3], [:months, -3]]`. This would mean
that it was subtly different than `2.months - 2.months`. When applied to
a time, the date might actually change if the resulting day doesn't
exist however many months in the future, even though in both cases we
just wanted to add `0`, which should always be an identity operation.

With this change, we now store the parts as a hash, so `3.months -
3.months` is simply stored as `{ months: 0 }`.

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