BF_EVACUATED is now set on all blocks except those that we are
copying.  This means we don't need a separate test for gen>N in
evacuate(), because in generations older than N, BF_EVACUATED will be
set anyway.  The disadvantage is that we have to reset the
BF_EVACUATED flag on the blocks of any generation we're collecting
before starting GC.  Results in a small speed improvement.