Support nested children

[theKashey]

This will solve #279 (not decorators, but test case).
The issue is very popular in react world

<Layout><A/><B/></Layout>
const Layout = ({children}) => <div><h1/>{children}</div>

The problem - children can be

1. A multy dimension array (React 15/16)
2. Object with children, and type:null (Enzyme???)

This PR solves a problem with merging complex structure as children, and able to reconcile decorated class in a complex environment.

[codecov-io]

Codecov Report

Merging #735 into next will increase coverage by 4.14%.
The diff coverage is 91.89%.

@@            Coverage Diff             @@
##             next     #735      +/-   ##
==========================================
+ Coverage   71.42%   75.57%   +4.14%     
==========================================
  Files          12       12              
  Lines         420      434      +14     
  Branches       92      105      +13     
==========================================
+ Hits          300      328      +28     
+ Misses        102       87      -15     
- Partials       18       19       +1

Impacted Files	Coverage Δ
...-hot-loader/src/reconciler/hotReplacementRender.js	87.73% <91.89%> (+10.54%)	⬆️
...ages/react-hot-loader/src/reconciler/reactUtils.js	84.61% <0%> (ø)	⬆️
...es/react-hot-loader/src/reconciler/reactHydrate.js	55.76% <0%> (ø)	⬆️

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