fix: Fix a rounding error at the Number::maxRep cusp

- Add helper function, doDropDigit, to wrap the common pattern:
    push(mantissa % 10);
    mantissa /= 10;
    ++exponent;
- Might have been helpful to catch this issue when developing.

src/test/jtx/impl/AMMTest.cpp

@@ -143,7 +143,7 @@ AMMTestBase::testAMM(std::function<void(jtx::AMM&, jtx::Env&)> const& cb, TestAM
         // mantissas
         Env env{
             *this,
-            features - featureSingleAssetVault - featureLendingProtocol,
+            features - featureSingleAssetVault - featureLendingProtocol - fixCleanup3_2_0,
             arg.noLog ? std::make_unique<CaptureLogs>(&logs) : nullptr};
 
         auto const [asset1, asset2] = arg.pool ? *arg.pool : std::make_pair(XRP(10000), USD(10000));