tune EF coding

tune EF coding

Summary:
Change the way forward / skip pointers are encoded, so we can
expect that `uint32_t` will be enough to address ~4B elements instead
of ~2B as it is now.

Test Plan:
Ran benchmarks for both versions, saw no significant
difference.

Added tests.

Reviewed By: lucian@fb.com

FB internal diff: D1156950