Refactor offset-allocator to improve readability

[patowen]

A few months ago, as part of my work on Hypermine, I was looking into this crate as something to potentially use to manage allocations in Vulkan. While evaluating this crate, I studied the code carefully to understand how it worked, and while doing so, I made some rather extensive refactors to make it easier for me to understand this code. Eventually, I was happy enough with the refactors that I was in favor of vendoring this dependency in Hypermine so that I could take advantage of these refactors.

However, it was suggested that I look into trying to contribute my refactor upstream. I was initially hesitant to do so because this crate was meant to be a port of a C++ package, which itself is considered an "early one weekend prototype", and it seemed like this crate is meant to be as faithful to that C++ package as well.

Regardless, I'm submitting this draft PR in case you are interested in seeing what I have so far, in case such a refactor is worthwhile to have upstream. If you are interested in such a PR, please let me know, and I can try to clean it up to make it easier to review (and ready for the library, as I still left a TODO in here).

This PR makes the following changes:

• Add documentation to better explain how the allocator works
• Use the slab crate for managing free node indexes (I may revert this part and use a different abstraction, as it's arguably more memory heavy)
• Set up a newtype for SmallFloat
• Create SmallFloatMap, which SmallFloat is used as an index for
• Create BinsMap as a layer of abstraction for used_bins etc.
• Set up a custom NodeIndexOption type to remove nonmax crate dependency

For further historical context on how these changes came to be, see these commit history of https://github.com/patowen/offset-allocator/commits/rewrite-wip/.