Change code generation to use variables to simulate the stack

[lucabol]

This is to help the optimizer, which works better with variables than memory access. The idea is to add new vars to represent new values pushed on the stack and keep somewhere the definition of which vars correspond to stack positions.

// push 2 on the stack
var x34 = 2
// modify a data structure to say that x34 is top of stack
stackpostions.add(34);