refactor: Payment component calculation will target next true state

- Compute the next "true" state, round the values off, then compute the
  deltas needed to get the current state to that state. Plus some data
  integrity checks.
- Add `Number::zero`, which is longer to type, but more readable than
  `Number{}`.
- Prepare to improve Loan unit tests: track managementFeeRate in
  BrokerInfo, define a LoanParameters object for creation options and
  start adding support for it, track and verify loan state while making
  payments.

include/xrpl/basics/Number.h

@@ -58,6 +58,8 @@ public:
     explicit Number(rep mantissa, int exponent);
     explicit constexpr Number(rep mantissa, int exponent, unchecked) noexcept;
 
+    static Number const zero;
+
     constexpr rep
     mantissa() const noexcept;
     constexpr int