`STNumber` lets objects and transactions contain multiple fields for
quantities of XRP, IOU, or MPT without duplicating information about the
"issue" (represented by `STIssue`). It is a straightforward serialization of
the `Number` type that uniformly represents those quantities.
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Co-authored-by: John Freeman <[email protected]>
Co-authored-by: Howard Hinnant <[email protected]>
