Added to _bitmask auto increment bit numbers

Now (_bitmask '(a b)) == (_bitmask '((a 1) b)) == (_bitmask '((a 1) (b 2))

Or (_bitmask '((a 8) b)) == (_bitmask '((a 8) (b 16)))

Or (_bitmask '((a 7) b)) == (_bitmask '(a = 7 b)) == (_bitmask '((a 7) (b 8)))

I hope, this will be useful