Make most implicit integer truncations and sign conversions explicit

Make most implicit integer truncations and sign conversions explicit

Summary:
This makes a large number of implicit truncations and sign conversions into explicit casts, in preparation for eventually enabling `-Wconversion` on Folly.
This set of changes should have zero semantic change.

This focuses on the core of Folly and leaves the tests alone for the most part. A future diff will cleanup the warnings in the tests as well.

Reviewed By: yfeldblum

Differential Revision: D4454096

fbshipit-source-id: af1e1831675a804ec5679266c30c1fae62e8c35c