Fix #192: Range: Re-enable the optimized version of Range<Integer>.by()
(where we can use  a + instead of iterating doing ++) but using a native
helper method. This is a hack until we have reified generics and can write the
optimization in pure Ceylon.
Add a test for the lengthwise range op (start:length)
Util: Add a method for creating the Range (or empty) for a start:length SpreadOp
(for ceylon/ceylon-compiler#836)