de: simplify implementation, remove dict keys allocation

[davidhewitt]

No description provided.

[codecov-io]

Codecov Report

Merging #11 (26e5951) into master (400d6f9) will increase coverage by 2.90%.
The diff coverage is 80.95%.

@@            Coverage Diff             @@
##           master      #11      +/-   ##
==========================================
+ Coverage   73.94%   76.84%   +2.90%     
==========================================
  Files           4        4              
  Lines         641      622      -19     
==========================================
+ Hits          474      478       +4     
+ Misses        167      144      -23     

Impacted Files	Coverage Δ
src/error.rs	10.95% <40.00%> (+10.95%)	⬆️
src/de.rs	88.26% <86.48%> (+0.37%)	⬆️

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