One read for b-tree nodes

[bnlawrence]

Description

This addresses b-tree read performance issues outlined in #153 by minimising the number of actual reads necessary to read a b-tree node. On some file systems this should make a big difference.

Closes #153

Checklist

• This pull request has a descriptive title and labels
• This pull request has a minimal description (most was discussed in the issue, but a two-liner description is still desirable)
• Unit tests have been added (if codecov test fails)
• Any changed dependencies have been added or removed correctly (if need be)
• If you are working on the documentation, please ensure the current build passes
• All tests pass

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.24%. Comparing base (5d3438a) to head (9a99dce).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #154      +/-   ##
==========================================
+ Coverage   76.18%   76.24%   +0.06%     
==========================================
  Files          14       14              
  Lines        2893     2901       +8     
  Branches      459      459              
==========================================
+ Hits         2204     2212       +8     
  Misses        564      564              
  Partials      125      125              

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[bnlawrence]

This has yet to be tested on real data and evaluated, but shared as a draft for evaluation and discussion.