Support React.Fragments. #799

[theKashey]

Add React.Fragment support. Literally just flatmap children out of the React.Fragment element.
Not to be merged, first I have to investigate why Fragment sometimes works as a host node (children next to instance), and sometimes as a class (children as prop).

[codecov-io]

Codecov Report

Merging #800 into next will increase coverage by 0.28%.
The diff coverage is 92.3%.

@@            Coverage Diff             @@
##             next     #800      +/-   ##
==========================================
+ Coverage   84.95%   85.23%   +0.28%     
==========================================
  Files          22       22              
  Lines         472      481       +9     
  Branches      129      133       +4     
==========================================
+ Hits          401      410       +9     
- Misses         53       59       +6     
+ Partials       18       12       -6

Impacted Files	Coverage Δ
...ckages/react-hot-loader/src/internal/reactUtils.js	92.3% <100%> (+11.35%)	⬆️
...-hot-loader/src/reconciler/hotReplacementRender.js	88.54% <90%> (-0.83%)	⬇️
...ot-loader/src/internal/stack/hydrateLegacyStack.js	8.69% <0%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8802f03...7923d79. Read the comment docs.