There are too many layers to the JSX runtime implementation. I think
basically everything should be implemented in a single file, so that's
what I'm going to do.
As a first step, this deletes ReactJSXElementValidator and moves all the
code into ReactJSXElement. I can already see how this will help us
remove more indirections in the future.
Next I'm going to do start moving the `createElement` runtime into this
module as well, since there's a lot of duplicated code.
Andrew Clark
ec160f32c2