Break out BitIterator into its own header

Break out BitIterator into its own header

Summary:
BitIterator is a standalone construct that has a heavy boost
dependency, but not common usage. Breaking it out into its own header
will help a lot of transitive dependendencies, because it is included
in Hash.h which is included in a variety of other common headers.

This reduces the number of transitively included headers by 248 (!)

Reviewed By: yfeldblum, ot, luciang

Differential Revision: D6178564

fbshipit-source-id: 1380154b012615b7b8c73bc15ab0ac62f6b990d3