Bitbase index() from ADD to OR.  No functional change.

I initially wrote this as a speed optimization but because Intel has done a great job making the ADD instruction with carry as fast as a bitwise OR it is exactly the same speed.  I'm submitting it anyway because from a purist point of view the bitwise OR better reflects that the index is a series of bitfields as documented in the comment section instead of a number arrived at through an algebraic expression.  Also it might still be faster on more primitive architectures.  I realize this is very nit picky so if you don't like it just close the pull request.  No functional change.