Add support for handling negative step values in ranges (fixes #500).
Add test cases for new range expression handling.
Update 'children' test case using range [4 : -1 : 0] which is now valid.
Fix output of range expressions.
Update 'for' test cases to match the new range expression handling.
Change nbsteps() to uint32_t to make it independent from 32/64-bit platform.
Merge pull request #542 from t-paul/issue500

Allow for statement with negative step value.