hilbert: implement zorder for archs other than x86_64

[hhirtz]

• for x86: use twice as many pdep32s and OR the results,
• for other archs: loop over the bits. I didn't measure the performance.

Don't need this for now, and fallback code compiles to a lot of instructions, so I'll keep this PR open

Fixes #260

TODO

• unit tests

[codecov]

Codecov Report

Base: 55.47% // Head: 56.04% // Increases project coverage by +0.57% 🎉

Coverage data is based on head (ef30337) compared to base (7461d81).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #273      +/-   ##
==========================================
+ Coverage   55.47%   56.04%   +0.57%     
==========================================
  Files          41       41              
  Lines        7530     7565      +35     
==========================================
+ Hits         4177     4240      +63     
+ Misses       3353     3325      -28     

Impacted Files	Coverage Δ
src/algorithms/hilbert_curve.rs	85.92% <100.00%> (+0.96%)	⬆️
src/algorithms/vn/first.rs	91.22% <0.00%> (+0.87%)	⬆️
src/algorithms/arc_swap.rs	84.70% <0.00%> (+10.07%)	⬆️

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[cedricchevalier19]

Related to #310

[cedricchevalier19]

Already done in #310