fold objects for list keys (proof-of-concept)

## _DO NOT MERGE_

This PR is just to track work, and make it easy to share with others.

## TODO

- [ ] Basic proof-of-concept
- [ ] Feature parity w/ s3/current-list keys (prefix, delimiter, max-keys, etc.)
- [ ] Delimiter key-skipping hack
- [ ] Eliminate code-duplication between new and old listing
- [ ] Switching of some sort based on capabilities for new and old code
- [ ] Profiling and observability info
- [ ] 5-node cluster benchmarks
- [ ] Check for memory leaks (at least a simple 'run this 1k times' and check for no memory growth)
- [ ] Pure EQC tests
- [ ] Share some of the old tests
- [ ] Make sure riak_test passes