Add support for React.memo

[bvaughn]

Remove React.pure (since it was never released in a stable) and add support for React.memo. Preserve previous custom displayName semantics.

Resolves #1193

Code

function CounterFunction({count}) {
  return <small>{count}</small>;
}
const A = React.memo(({count}) => <small>{count}</small>);
const B = React.memo(CounterFunction);
const C = React.memo(CounterFunction);
C.displayName = 'MemoCounterFunction';

DevTools

[bvaughn]

I'm just going to close this and roll everything into #1207 for simplicity.