Return keys from denormalize

[jonaskello]

Fixes #30.

This PR will add the keys of the normalized objects used during denormalization to the output of denormalize().

[codecov]

Codecov Report

Merging #36 into master will increase coverage by 0.11%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #36      +/-   ##
==========================================
+ Coverage   87.61%   87.72%   +0.11%     
==========================================
  Files           7        7              
  Lines         218      220       +2     
  Branches       56       56              
==========================================
+ Hits          191      193       +2     
  Misses         12       12              
  Partials       15       15

Impacted Files	Coverage Δ
src/denormalize.ts	95.45% <100%> (+0.14%)	⬆️

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