fix high cpu usage

[FranciscoCaetano88]

Due to the high CPU demand of executing a step function per code expression with a given timeout, we decided to only use the timeout in certain cases. We need to keep the steps per code expression so we can pause/resume/stop the interpreter.

We now have 4 step cases:

• step() => resolve immediately
• step(0) => resolve immediately
• step(100) => resolves in 100ms
• step(Infinity) => doesn't resolve (used when the stepper is paused, it'll resolve when the stepper is resumed)

Steps should resolve as fast as possible so we removed the step time option from the project.
Some tests required some changes.