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/app/Discrepancy_test.cpp

@@ -41,7 +41,7 @@ class Discrepancy_test : public beast::unit_test::suite
     {
         testcase ("Discrepancy test : XRP Discrepancy");
         using namespace test::jtx;
-        Env env {*this, features(fs)};
+        Env env {*this, with_features(fs)};
 
         Account A1 {"A1"};
         Account A2 {"A2"};