Make Range.h and FBString.h mutually independent

Make Range.h and FBString.h mutually independent

Summary:
[Folly] Make `Range.h` and `FBString.h` mutually independent.

This means that `Range` cannot directly know about `fbstring`, so any interactions between the two types must be indirected through templates.

Motivation: `FBString.h` is a relatively heaviweight `#include` for things that need `Range.h` but which do not use `fbstring`.

Reviewed By: ericniebler

Differential Revision: D6062434

fbshipit-source-id: e2f21c33f482eadffd0a8679eff4ece59bab53e9