Range Iterator

Build RangeIterator(start, end, step=1), an iterator over integers that behaves like Python's range but is written by hand (don't call range or itertools inside it).

• It produces start, start + step, start + 2*step, … and stops before reaching end (end is exclusive).
• A positive step counts upwards and stops once a value would be >= end; a negative step counts downwards and stops once a value would be <= end.
• If the direction of step can never reach end (for example start=5, end=1, step=2), the iterator is simply empty.
• step == 0 raises ValueError in the constructor.

Methods:

• has_next() -> bool: is there another value?
• next() -> int: return the next value and advance. Raise StopIteration when exhausted.
• remaining() -> int: how many values are still to come, in O(1).
• It must also work in a for loop: implement __iter__ (returning self) and __next__.

it = RangeIterator(2, 11, 3)
it.remaining()     # 3
it.next()          # 2
list(it)           # [5, 8]
it.has_next()      # False

list(RangeIterator(10, 0, -4))   # [10, 6, 2]
list(RangeIterator(-3, -3))      # []
list(RangeIterator(5, 1, 2))     # []

Values can be as large as 10**18, and a range may hold that many values: the iterator must be lazy (constant memory, O(1) per call).