HitSet: enable HitSet::operator=

Signed-off-by: Greg Farnum <greg@inktank.com>
fixup Hitset: do fpp_micro math in the proper direction

Signed-off-by: Greg Farnum <greg@inktank.com>
WIP test: initial HitSet unit test

Signed-off-by: Greg Farnum <greg@inktank.com>