Use OnceLock in place of RwLock

[macladson]

Addresses #96

Swap out the RwLock for a OnceLock for lock-free reads

[macladson]

Interesting. Seems like MacOS is doing padding differently to Linux

[codecov-commenter]

Codecov Report

❌ Patch coverage is 80.28169% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.65%. Comparing base (3fcb03c) to head (7b1cfdd).

Files with missing lines	Patch %	Lines
src/tree.rs	78.26%	10 Missing ⚠️
src/packed_leaf.rs	81.81%	2 Missing ⚠️
src/leaf.rs	75.00%	1 Missing ⚠️
src/utils.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #97      +/-   ##
==========================================
- Coverage   70.15%   69.65%   -0.51%     
==========================================
  Files          22       22              
  Lines        1280     1249      -31     
==========================================
- Hits          898      870      -28     
+ Misses        382      379       -3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.