Move towards N-ary OP implementation
Inline $x + const into JIT function

Yay, first fully inlined OP. It's still included in the basic
representation of the OP tree, but it's wired out of the execution
thread. Unfortunately, the latter also means that "const + $x" doesn't
JIT the const OP yet since the incoming op_next isn't as easy to rewire.
no asserts without debug mode