Fix invalid HardenedIndex::ONE constant

[dr-orlovsky]

Closes #38

[codecov]

Codecov Report

Attention: Patch coverage is 21.42857% with 11 lines in your changes missing coverage. Please review.

Project coverage is 42.5%. Comparing base (04a7a2b) to head (e1df0fc).
Report is 2 commits behind head on master.

Files	Patch %	Lines
derive/src/index.rs	21.4%	11 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master     #39     +/-   ##
========================================
- Coverage    42.6%   42.5%   -0.0%     
========================================
  Files          24      24             
  Lines        4233    4232      -1     
========================================
- Hits         1802    1800      -2     
- Misses       2431    2432      +1     

Flag	Coverage Δ
rust	42.5% <21.4%> (-<0.1%)	⬇️

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[zoedberg]

tACK e1df0fc