Summary:
Implicitly bouncing between size_t and int for identifiers
causes problems when building on 64-bit iOS. Fix this.

Test Plan: fbmake runtests

Reviewed By: meyering@fb.com, njormrod@fb.com

Subscribers: trunkagent, njormrod, folly-diffs@, fma, kmdent, shikong, pgriess

FB internal diff: D1722061

Signature: t1:1722061:1417819039:8e3938cf8d4d241551ed3dd3978c1b11f57398c5