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/LedgerRPC_test.cpp

@@ -286,7 +286,7 @@ class LedgerRPC_test : public beast::unit_test::suite
     void testLookupLedger()
     {
         using namespace test::jtx;
-        Env env {*this, no_features}; // hashes requested below assume
+        Env env {*this, FeatureBitset{}}; // hashes requested below assume
                                      //no amendments
         env.fund(XRP(10000), "alice");
         env.close();