Introduce concept of elements as distinct from components

[myitcv]

!! BREAKING CHANGE !!

One of the shortcuts I had taken until this point was to conflate the concept of a component (definition) and an element. This PR introduces the concept of an element. More precise definitions to follow in the wiki, but briefly:

• a component is as it was before, and corresponds directly to a React component. It is the definition, potentially with props and state, of a thing that Render's
• an element is an "instance" of a component (this definition needs to improve... to mention mounting etc)

This has the nice side effect of resolving many of the spurious re-Render's that were happening.

So changes in this PR include:

• Introduced elements and refactored existing components to reflect that change. For core components, e.g. the component behind the <div> HTML element, we no longer declare *Def component; instead the constructor functions, e.g. react.Div(...) return an element, in this case react.DivElement
• react.BlessElement is now react.CreateElement
• Declaring your own component, e.g. MyCompDef, requires that methods be declared on the type MyCompDef and not *MyCompDef (docs to follow)
• Various refactors of methods in myitcv.io/react

Less interesting:

• Vendored github.com/gopherjs/jsbuiltin