Constructing an Amount from an Amount?

In the comments for #49 the question of whether we should be able to construct an Amount from an Amount arose, which seems worth asking outside of the context of that issue.

So: bracketing off the questions related to using with to increase the number of significant digits that were raised in that issue, should we treat:

let a = new Amount(1.234, { significantDigits: 4 });
let b = new Amount(a, { significantDigits: 2 });

as equivalent to

let b = new Amount(1.234, {significantDigits: 4}).with({ significantDigits: 2 })

and, more generally, should we allow Amounts as arguments to the Amount constructor?

Start by reviewing the discussion in issue #49 and the examples in this issue. Determine whether the Amount constructor should accept an existing Amount and how significantDigits should behave; the work is complete only once the project’s API decision is documented and any corresponding implementation or tests are identified.