WIP: AST Interpreter
Bash the existing OSR code into submission for the new AST interpreter

Once we get the AST interpreter in, we can add resume-from-any-point
functionality, which means that we can get rid of the current OSR system.
Maybe we should have done that first, but I thought it would be easy
to get the OSR stuff working.
AST interpreter cleanup

- Move internal definitions from header to source file
- Fix root_stack_set bug carried over from llvm_interpreter
- Cut off access to the old llvm interpreter
- Slightly tune the tiering thresholds
 - They seem too high but our LLVM compilation is overly expensive right now
 - Our AST interpreter is also somewhat slow due to its string handling
Merge pull request #228 from kmod/interp2

New AST interpreter