Aalekh Patel

Okay! There's some careful copypasta work in #79 that covers the base `BinaryIndexImpl` and all the core `*Index*` traits and macros are replicated and adapted for `BinaryIndex`es. Not so sure...

On a second thought, I'll keep the naming convention the same as the C++ api (i.e. `IndexFlat` -> `IndexBinaryFlat`)

Thanks for your work! Wondering if there are any blockers for landing this atm. Let me know if I can be of any help.

Thank you for your prompt feedback. I hope I'm not adding too much to your plate. Please feel free to take your time with the review. I'm glad I could...