feefrac: drop comparison and operator{<<,>>} for sorted wrappers

Instead of having an unintuitive but total implicit sort order on
FeeFrac (first increasing feerate, then decreasing size), and separate
overloaded operator<< and operator>> for a weak ordering that only looks
at feerate, replace these with explicit wrapper classes which make the
behavior more explicit.

This allows for things like ByRatio{a} <= ByRatio{b}, instead of the
earlier !(a >> b). It also supports usage inside std::max and
std::greater, so one can use:
* std::max<ByRatioNegSize<FeeFrac>>(a, b)
* std::sort(v.begin(), v.end(), std::greater<ByRatioNegSize<FeeFrac>>{})

src/node/mini_miner.cpp

@@ -184,12 +184,12 @@ struct AncestorFeerateComparator
         auto min_feerate = [](const MiniMinerMempoolEntry& e) -> FeeFrac {
             FeeFrac self_feerate(e.GetModifiedFee(), e.GetTxSize());
             FeeFrac ancestor_feerate(e.GetModFeesWithAncestors(), e.GetSizeWithAncestors());
-            return std::min(ancestor_feerate, self_feerate);
+            return std::min<ByRatioNegSize<FeeFrac>>(ancestor_feerate, self_feerate);
         };
         FeeFrac a_feerate{min_feerate(a->second)};
         FeeFrac b_feerate{min_feerate(b->second)};
         if (a_feerate != b_feerate) {
-            return a_feerate > b_feerate;
+            return ByRatioNegSize{a_feerate} > ByRatioNegSize{b_feerate};
         }
         // Use txid as tiebreaker for stable sorting
         return a->first < b->first;