Fix containers and out of order tables dict behavior

[sdispater]

Fixes #80
Fixes python-poetry/poetry#2110
Supersedes #81

[codecov-io]

Codecov Report

Merging #82 into master will increase coverage by 0.12%.
The diff coverage is 90%.

@@            Coverage Diff             @@
##           master      #82      +/-   ##
==========================================
+ Coverage   91.11%   91.23%   +0.12%     
==========================================
  Files          10       10              
  Lines        2206     2225      +19     
==========================================
+ Hits         2010     2030      +20     
+ Misses        196      195       -1

Impacted Files	Coverage Δ
tomlkit/container.py	85.56% <90%> (+0.82%)	⬆️

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