Use Rate to represent transfer rates (RIPD-201, RIPD-983):

The Ripple protocol represent transfer rates and trust line
qualities as fractions of one billion. For example, a transfer
rate of 1% is represented as 1010000000.

Previously, such rates where represented either as std::uint32_t
or std::uint64_t. Other, nominally related types, also used an
integral representation and could be unintentionally substituted.

The new Rate class addresses this by providing a simple, type
safe alternative which also helps make the code self-documenting
since arithmetic operations now can be clearly understood to
involve the scaling of an amount by a rate.

src/ripple/app/paths/impl/DirectStep.cpp

@@ -546,8 +546,9 @@ DirectStepI::qualities (
     {
         // Charge a transfer rate when issuing and previous step redeems
         auto const prevStepRedeems = prevStep_ && prevStep_->redeems (sb, fwd);
-        std::uint32_t const srcQOut =
-            prevStepRedeems ? rippleTransferRate (sb, src_) : QUALITY_ONE;
+        std::uint32_t const srcQOut = prevStepRedeems
+            ? transferRate (sb, src_).value
+            : QUALITY_ONE;
         return std::make_pair(
             srcQOut,
             quality ( // dst quality in