ENH: slicing with decreasing monotonic indexes

Fixes #7860 

The first commit adds `Index.is_monotonic_decreasing` and `Index.is_monotonic_increasing` (alias for `is_monotonic`).

`is_monotonic` will have a performance degradation (still O(n) time) in cases where the Index is decreasing monotonic. If necessary, we could work around this, but I think we can probably get away with this because the fall-back options are much slower and in many cases (e.g., for slice indexing) the next thing we'll want to know is if it's decreasing monotonic, anyways.

Next up will be handling indexing monotonic decreasing indexes with reversed slices, e.g., `s.loc[30:10]`.

CC @jreback @hugadams @immerrr (thank you, specialities wiki!)
see `test_is_monotonic_na`