omit nextstate COPs

[rurban]

nextstate resets the stack at every ; which is a waste and adds enormous run-time costs.
Track the stack depth for each op result if statically possible, add the oplines field to each op, thus get rid of most nextstate COPs.

nextstate is one of the most costly ops (see dtrace Porting/op.d), and is called every single line.

stack tracking+oplines. see the jit #220