Eliminated SmallIntSet

SmallIntSet is equivalent to BitvSet but with 8 times the memory
overhead. There's no reason for it to exist.
auto merge of #7934 : sfackler/rust/smallintset, r=alexcrichton

SmallIntSet is equivalent to BitvSet but with 64 times the memory
overhead. There's no reason for it to exist.

SmallIntSet's overhead should really only be 8 times, but for some
reason, `sys::size_of::<Option<()>>() == 8`, not 1.