PERF: RangeIndex.is_monotonic_inc/dec
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

as the same as `is_unique`, it should be monotonic by definition

```
%timeit -n 1 -r 1 pd.RangeIndex(0, 1000000).is_monotonic_increasing

# current master
#1 loop, best of 1: 17.2 ms per loop

# this PR
#1 loop, best of 1: 45.1 µs per loop
```
