Support loops of varying length

[gmile]

Fixes #56

This code does not yet have tests. It does not account for the following corner cases:

• a zero is passed as any of range Loop arguments,
• max is less than min, e.g. range Loop 10 5 – this case is not handled,
• a negative number is passed as any of range Loop arguments (I think it makes sense to use uint everywhere in range Loop),
• a non-digit is passed as any of range Loop arguments,
• three or more arguments are passed to range Loop (in fact, in case more than 2 args are passed – redundant args will be ignored),
• an integer more than MaxInt is passed as any of range Loop arguments,
• what will be if min = max?