Change features default behavior in Env (RIPD-1460):

Enable all supported amendments in Env by default. Rename `features()`
to `with_features()` and add `all_features_except()` to support feature
subsets in Env. Refactor internal feature handling based on a bitset.

src/test/rpc/GatewayBalances_test.cpp

@@ -34,7 +34,7 @@ public:
     {
         using namespace std::chrono_literals;
         using namespace jtx;
-        Env env(*this, features(fs));
+        Env env(*this, with_features(fs));
 
         // Gateway account and assets
         Account const alice {"alice"};