Summary:
[Folly] Mark `BitIterator<BaseIterator>` with the iterator category of `BitIterator`.

As one benefit, using `std::distance` to find the distance between two `BitIterator`s gotten from a random access container such as `std::vector` is now constant-time.

Fixes #1026.

Reviewed By: ot

Differential Revision: D14294852

fbshipit-source-id: 2345bc73dec169803ae41b9391687e89ad77207b