Refactor FeatureBitset:

* Remove composite helper functions
* Add set difference and Bitset/uint256 operators
* Convert tests to use new feature bitset set difference operator

src/test/rpc/LedgerData_test.cpp

@@ -267,8 +267,9 @@ public:
         // Put a bunch of different LedgerEntryTypes into a ledger
         using namespace test::jtx;
         using namespace std::chrono;
-        Env env { *this, envconfig(validator, ""),
-            supported_features_plus(featureTickets) };
+        Env env{*this,
+                envconfig(validator, ""),
+                supported_amendments().set(featureTickets)};
 
         Account const gw { "gateway" };
         auto const USD = gw["USD"];