PERF: Avoid MultiIndex conversion for IntervalIndex methods
There are a few `IntervalIndex` methods that convert to a `MultiIndex` as an intermediate step, and then use the associated `MultiIndex` method to compute the result.  This likely introduces overhead that could be avoided via a more direct `IntervalIndex` implementation.

Methods that currently require a `MultiIndex` conversion:
- [x] `IntervalIndex.is_monotonic` (#25820)
- [x] `IntervalIndex.is_monotonic_increasing` (#25820) 
- [x] `IntervalIndex.is_monotonic_decreasing` (#25820)
- [x] `IntervalIndex.is_unique` (#26391)
- [ ] <s>`IntervalIndex._get_loc_only_exact_matches`</s> Update: no longer exists
- [ ] `IntervalIndex.union`
- [x] `IntervalIndex.intersection` (#26225)
- [ ] `IntervalIndex.difference`
- [ ] `IntervalIndex.symmetric_difference`
