Fix 64 bit BitArray bugs + BitArray.length bug

Fixes:
http://d.puremagic.com/issues/show_bug.cgi?id=11072
http://d.puremagic.com/issues/show_bug.cgi?id=11015
http://d.puremagic.com/issues/show_bug.cgi?id=10948
Plus a BitArray.length bug discovered during code review.