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.
When started with "--start", put all known, non-vetoed
amendments in the genesis ledger. This avoids the need
to wait 256 ledgers before amendments are enabled when
testing with a fresh ledger.
When attempting to parse a BufferSequence as a JSON object,
if the sequence contained more than buffer, the JSON parser
would incorrectly attempt to decode each buffer as a separate
JSON object, instead of one complete object.
