Implement variable iterable ranges

[LefterisJP]

At the moment in refu we can only have a range with constant numbers such as:

for a in 1:2:10 {
}

Implement variable ranges so that the range start, step and end can also be an identifier. For example the following should also be possible:

for a in b:2:c {
}

[LefterisJP]

This is now done but only up to the typechecking stage. Remaining is the to generate the backend code, tracked by: #28