os-macosx.mm: Use uname() instead of Gestalt for version check. Allow 10.5 and over still. Fixes #585. Fixes #815. Thanks @erikcharlebois and @jckarter for the motivation and the fix.
sets: encourage re-use by adding (intersect) and (diff).
hash-sets: simpler code.
sets: inline small/large.
hash-sets: faster intersects?.
hash-sets: faster subset? and set= when both are hash-sets.
hash-sets: faster intersect and diff, when both are hash-sets.
hash-sets: slightly faster filter-members.
benchmark.hash-sets: do more benching (create sets in a literal).

Don't run the "intersects?" test on old hash-sets, they are sloooooow.
hash-sets: inline >hash-set to speed up set operations.