ENH: Avoid double-loops in _RecursiveMappingView

[BvB93]

They're very slow due to the lazy nature of the class, so instead operate via their set-based counterpart.

[codecov]

Codecov Report

Merging #37 (1534bf9) into master (cd0a4cf) will increase coverage by 0.08%.
The diff coverage is 63.79%.

❗ Current head 1534bf9 differs from pull request most recent head 2ed99ac. Consider uploading reports for the commit 2ed99ac to get more accurate results

@@            Coverage Diff             @@
##           master      #37      +/-   ##
==========================================
+ Coverage   85.36%   85.44%   +0.08%     
==========================================
  Files          18       18              
  Lines        1319     1347      +28     
  Branches      209      208       -1     
==========================================
+ Hits         1126     1151      +25     
+ Misses        152      147       -5     
- Partials       41       49       +8     

Impacted Files	Coverage Δ
nanoutils/hdf5_utils.py	83.57% <61.53%> (-13.24%)	⬇️
nanoutils/_lazy_import.py	88.99% <83.33%> (+11.82%)	⬆️

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