AST: keys/values on %/@/ref
AST: once OP
Add first Array type shortcut
Add FIXME
Initial support for JITting C-style for loops

Does not handle next/last/redo (and goto...).
Make last_op() work after JITting, when the unstack op has been detached
Replace generic pa_pp_op call with pa_pp_unstack
Replace generic pa_pp_op call with pa_pp_nextstate
Support for JITting while/until loops

Does not support do {} while/until loops (requires being able
to JIT blocks).