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
```

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13749?src=pr) is 84.57% (diff: 100%)

> No coverage report found for **master** at b60e42b.
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