WIP: slicing with decreasing monotonic indexes

Fixes #7860 

The first commit adds `Index.is_monotonic_decreasing` and `Index.is_monotonic_increasing` (alias for `Index.is_monotonic`).

Next up will be handling indexing monotonic decreasing indexes with reversed slices, e.g., `s.loc[30:10]`. There will definitely be a small amount of API breakage there (the later currently returns an empty array).

CC @jreback @hugadams @immerrr (thank you, specialities wiki!)