Ensure proper insert order of nested rules & atRules

[tkh44]

What:

Why:

How:
Here is where I played around with this idea. https://codesandbox.io/s/kxxvqxrxnv

Checklist:

• Documentation
• Tests
• Code complete

[emmatown]

I was originally planning to do this but the insert order problem affects nested selectors as well as media queries. https://codesandbox.io/s/p5ymkr9n0

[codecov]

Codecov Report

Merging #321 into master will increase coverage by 0.08%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #321      +/-   ##
==========================================
+ Coverage   96.29%   96.37%   +0.08%     
==========================================
  Files          16       16              
  Lines         567      580      +13     
  Branches      138      141       +3     
==========================================
+ Hits          546      559      +13     
  Misses         16       16              
  Partials        5        5

Impacted Files	Coverage Δ
packages/emotion/src/index.js	99.18% <100%> (+0.09%)	⬆️

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