[PROPOSAL] Iterator Improvements (Step, Reverse, Float)

I'd like to see more functionality for iterators, inspired by `Ruby`.

```haxe
// Standard IntIterator
for (i in (0...10))
trace(i); // 1, 2, 3, 4, 5...

// PROPOSALS

// Reverse Iterator
for (i in (10...0))
trace(i); // 10, 9, 8, 7, 6

// Step Iterator
for (i in (0...10).step(2))
trace(i); // 0, 2, 4, 6, 8

// FloatIterator with Step
// Compiler type-checks min and max to determine if it is an Int or a Float before choosing an iterator.
for (i in (5.0...7.5).step(0.1))
trace(i); // 5.0, 5.1, 5.2, 5.3, 5.4, ...
```

I was actually able to implement `step(value:Int):IntIterator` locally without modifying compilation, but reverse and FloatIterator would require compiler changes (I got a "Cannot iterate backwards" error)

This is pre-draft, but if nobody has any obvious problems with it I will write up a more formal proposal