Summary:
Add tests where the list size is close or at the maximum supported size, by fixing `EliasFanoReader` to support `uint16_t` as value and size type.

Types smaller than `int` require special support because any arithmetic operation promotes them to `int`, so operations that rely on unsigned overflow break. Work around this by introducing a helper for addition that preserves the type.

Reviewed By: philippv, luciang

Differential Revision: D32130076

fbshipit-source-id: 763a8e4cc2b023405c173316d2415ca8c3ec7148