* Remove composite helper functions
* Add set difference and Bitset/uint256 operators
* Convert tests to use new feature bitset set difference operator
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.
Add envconfig test helper for manipulating Env config via
callables. Create new common modifiers for non-admin config,
validator config and one for using different server port values.