Add BitSet and, as a test, convert some JitRegCache stuff to it.

This is a higher level, more concise wrapper for bitsets which supports
efficiently counting and iterating over set bits.  It's similar to
std::bitset, but the latter does not support efficient iteration (and at
least in libc++, the count algorithm is subpar, not that it really
matters).  The converted uses include both bitsets and, notably,
considerably less efficient regular arrays (for in/out registers in
PPCAnalyst).

Unfortunately, this may slightly pessimize unoptimized builds.
Convert registersInUse to BitSet.
Add workaround for OS X symbol clash and AllOnes helper method.
Convert some VideoCommon stuff to BitSet.

Now with a minor performance improvement removed for no reason.
Add unit test.
Merge pull request #1307 from comex/bitset

Higher level bitset wrapper