Remove unnecessary HashMap operations

[amoskvin]

This produces a 7-10% boost in read performance

[codecov[bot]]

Codecov Report

Merging #114 into master will decrease coverage by 0.08%. The diff coverage is 96.29%.

@@            Coverage Diff             @@
##           master     #114      +/-   ##
==========================================
- Coverage   86.15%   86.06%   -0.09%     
==========================================
  Files          25       25              
  Lines        1921     1916       -5     
==========================================
- Hits         1655     1649       -6     
- Misses        266      267       +1     

Impacted Files	Coverage Δ
src/extension/dublincore.rs	58.97% <94.73%> (+0.20%)	:arrow_up:
src/extension/util.rs	97.82% <100.00%> (-0.33%)	:arrow_down:

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[andy128k]

LGTM!