Parser priorities2

Don't review it. I'm just trying to test it.
To make them immutable, I need the data structures that @yilongli is working on right now.
I'm not done with changing the classes of the parser AST.
I still have to replace KInjectedLabel with Constant. I have to remove the quadratic behavior of appending parsing results to the current work list.

I know it's not completely correct, but I want to close this pull request. It's been a month now, and I want to switch to  unified-builtins.