React Renderer: refactor the code, clean it up and make it more generic

[mikehearn]

This series of commits is functionally nearly identical to the current code (save for a bug fix to the sandbox that nobody uses yet). However it improves the code in these ways:

1. It's smaller and simpler now.
2. The utils sub-package contains classes that could be integrated into Micronaut Core. They aren't React, Graal or Views specific.
3. The truffle sub-package contains classes that could be re-used for a more generic Truffle integration.
4. More objects are overridable via factories, and beans are now properly namespaced using qualifiers.
5. The docs for how to use hot-reload have been improved.

I hope that in future the generic code can find a new home in other modules.