Return modifiable objects

[MarcelWaldvogel]

Remove readonly attributes throughout the returned object tree

Fixes #30

[mcollina]

Can you update the test too: https://github.com/davidmarkclements/rfdc/blob/master/index.test-d.ts

[codecov]

Codecov Report

Merging #31 (a8b8aef) into master (6232d1d) will not change coverage.
The diff coverage is n/a.

❗ Current head a8b8aef differs from pull request most recent head dcca8c2. Consider uploading reports for the commit dcca8c2 to get more accurate results

@@            Coverage Diff            @@
##            master       #31   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines          136       136           
=========================================
  Hits           136       136           

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