Add sequence units to explainer

[eemeli]

This adds what I understand to be our current consensus on how to support sequence units (see e.g. #118 and proposal-intl-sequence-units), i.e. using an array as the .value where appropriate.

The spec is not (yet) updated by this change, just the explainer.

Going through the details, there are a few cases that I'm not sure we've covered previously, and on some of which this PR is opinionated:

• Combining significantDigits with sequence units — I recommend we throw an error for this, as the general solution here would effectively require dealing with non-decimal bases.
• Type of not-smallest unit values when applying fractionDigits — Are all array values stringified? For example, with

  let a = new Amount([5, 11.1], { fractionDigits: 0 })
  a.value  //  [5, '1.1e+1']  or  ['5e+0', '1.1e+1']

  both options seem somewhat reasonable to me.
• The non-localized serialization of not-smallest unit values — we require these to always be integers, so shouldn't we also serialize them as plain integers? Without significantDigits, these can't have precision information attached to them.
• As with the -per- units mentioned in #140, conversion to/from -and- units requires multiple sequential Number operations that can introduce deviations compared to decimal math.

Ping @sffc, @gibson042, @ljharb

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[jessealama]

Thanks! I agree that trying to specify significant digits when working with sequence units should throw, at least for now. I think it's worth discussing some of your other opinionated bits in new issues.