Don't allow modifications of the bound or step on a bounded iteration

[aitorres]

Just like we protect the iteration variable on a bounded iteration, the step and bound (stop) must be protected as well. We can either add some level of protection on the front-end (through Parsing, though this might be difficult because bounds and steps can be reduced as many different expressions) or through a hard copy of their values right before the execution, reusing the temporal, on TAC.