folly/Bits.h (BitIterator): avoid -Wsign-compare error

folly/Bits.h (BitIterator): avoid -Wsign-compare error

Summary:
* folly/Bits.h (BitIterator) [bitOffset_]: Change type of this
member from ssize_t to "unsigned int".
(bitsPerBlock): Change return type from size_t to "unsigned int".
These changes avoid some -Wsign-compare-highlighted issues, e.g.,
folly/Bits.h:456:18: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]

Test Plan:
Run this and note there are fewer errors than before:
fbconfig --platform-all=gcc-4.9-glibc-2.20 -r folly && fbmake dbgo

I've also run "fbmake runtests", and it failed precisely the way it does
without this patch.

Reviewed By: andrei.alexandrescu@fb.com

Subscribers: trunkagent, net-systems@, folly-diffs@

FB internal diff: D1770613

Tasks: 5941250

Signature: t1:1770613:1420757397:7ee7e5de500c8991af2597a6245d12d8f286b4b7