CLN: remove fastpath & verify_integrity from constructors
this is a top-level issue to remove the ``fastpath`` and/or ``verify_integrity`` keyword args from constructors from public interfaces, classes such as ``Categorical``,  ``Series``,  and``MultiIndex``. If these are removed all once from a particular class, e.g. ``Index/MultiIndex`` for ``verify_integrity``, but are still needed for an internal API, then can add an internal constructor ``_from_fastpath`` which can serve instead (e.g. see ``pandas.core.dtypes.dtype.CategoricalDtype`` for an example of this) There may be some re-factoring required to keep things DRY.

Most of these will actually need to be deprecated.

- [x] ``Index`` (fastpath)
- [ ] `Index` (verify_integrity in subclasses)
- [ ] ``MultiIndex`` (verify_integrity)
- [ ] ``pd.concat/.append`` (verify_integrity)
- [ ] `` Series`` (fastpath)
- [ ] ``Categorical`` (fastpath)