Complete React 16 support

Overarching project: https://github.com/airbnb/enzyme/projects/4

• Error Boundaries
  • componentDidCatch
    • #1255
  • Improve filenames in stack (https://github.com/facebook/react/pull/12059)
  • getDerivedStateFromError (#2036)
• Portals
  • #1710
  • #1507
  • #252
• React.Fragment
  • #1503
  • #1733
  • #1799
• Rendering non-jsx
  • #1149
  • #1178
  • #1213
  • #1498
• General issues around our use of private APIs
  • #1512
• New context APIs in 16.3
  • #1509
  • #1513
  • #1647
  • #1737
  • #1908
  • contextType (16.6) (#2189, upstream PR)
  • better support for render props (#1863)
• forwardRef
  • #1604
  • #1592
  • #1810 / #1817
• createRef
  • #1704
• <React.StrictMode />: https://reactjs.org/docs/strict-mode.html
  • include mode in component stack (https://github.com/facebook/react/pull/13240)
• <React.unstable_AsyncMode /> / <React.unstable_ConcurrentMode />:
• lazy/Suspense (16.6, #1917)
• memo (16.6, #1875)
• hooks (16.8, #1938)
• Profiler (#2054, #2233)