Kiddo 6.0.0-alpha.1

[sdd]

This PR is to introduce the first alpha of Kiddo 6. 10 months in the making and counting, it is effectively a full rewrite, addressing some long-standing issues.

• v6 represents a fundamental shift to a unified single KdTree struct, replacing the previous mutable/immutable and float/fixed splits.
• The macro-based query code that was used for v5 to permit more code reuse between plain and rkyv queries has been eliminated.
• The blanket impls for Axis from v5 have been removed, allowing Axis to now be implemented for floats, fixed point types, and unsigned ints. This removed the need for separate Axis and AxisFixed traits which drove the previous v5 requirement for separate float and fixed tree structs.
• The need for separate mutable / immutable trees has been removed by introducing the LeafStrategy trait, which the KdTree has as a generic property. LeafStrategies can be mutable or immutable.
• KdTree is also now generic over the new StemStrategy trait too. This allows experimentation and selection of alternative stem layouts and traversal mechanisms, orthogonal to the choice of leaf strategy.

At the moment v6 is still not quite ready for production use cases but I'm hoping to release it soon as an alpha so that people are able to try it out if interested and provide feedback, bug reports, and performance reports, to help get it to the point where it is production-ready. I'll be updating the benchmarking suite too with results from v6, as well as updating the stale results from other libraries to use their latest versions. I may add some new libraries against which to run the benchmarks too (suggestions welcome).

[codecov]

Codecov Report

❌ Patch coverage is 96.69118% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.34%. Comparing base (11870f8) to head (f23163f).

Files with missing lines	Patch %	Lines
src/dist/distance_metric_core.rs	83.60%	10 Missing ⚠️
src/dist/mod.rs	97.04%	8 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #301       +/-   ##
===========================================
- Coverage   94.88%   81.34%   -13.54%     
===========================================
  Files          54       70       +16     
  Lines        5705    14689     +8984     
  Branches     5705    14689     +8984     
===========================================
+ Hits         5413    11949     +6536     
- Misses        274     2651     +2377     
- Partials       18       89       +71     

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