[RFC] order-preserving key encoding

[patrick-schultz]

After discussing with @ehigham, we decided to try a lighter-weight RFC process, where an RFC is simply a PR for a new dev doc. Discussion about the design can happen here, as well as on the dev doc itself (typos, clarity, etc.). When approved and merged, it will be a dev-doc marked as an unimplemented RFC. When implemented, the RFC mark will be replaced by a reference to the implementing commit.