feat: A fully featured btree implementation

[wyhaines]

This is a fully featured btree implementation. A friend gave me some incomplete (and broken) btree code for Go, and when I started reworking it, I discovered that it was a broken semi-copy of an old version of Google's btree for Go.

I finished reworking it so that it adhere's to that original Google version's API, though there are some differences internally in places, and I think that my version is much easier to follow and to understand.

This implementation is quite a bit faster than the AVL tree. I will add links to some benchmarks that I did in a comment. This implementation supports copy-on-write for the trees, for inexpensively creating copies of a tree that are effectively isolated from each other with respect to changes that happen after the fork.

Contributors' checklist...

• Added new tests, or not needed, or not feasible
• Provided an example (e.g. screenshot) to aid review or the PR is self-explanatory
• Updated the official documentation or not needed
• No breaking changes were made, or a BREAKING CHANGE: xxx message was included in the description
• Added references to related issues and PRs
• Provided any useful hints for running manual tests

[n0izn0iz]

Thanks! I need this for that https://github.com/TERITORI/teritori-dapp/blob/2a5c8390438e6aad332f71410bcf8aabaaecab4b/gno/p/havl/havl.gno#L18
:D

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.79%. Comparing base (bd1d76e) to head (83a406e).
Report is 6 commits behind head on master.

Additional details and impacted files

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##           master    #3126   +/-   ##
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  Coverage   63.79%   63.79%           
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  Files         549      549           
  Lines       78819    78819           
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  Hits        50279    50279           
  Misses      25150    25150           
  Partials     3390     3390           

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