Rename Integer trait `divides` to `is_multiple_of`.

It is being changed because the previous wording was ambiguous.
`a.divides(b)` implied `a % b == 0` but it sounds like the other way
around. `9.divides(&3) == true` but we might read that as
"does 9 divide 3?".  It has been renamed to sidestep the ambiguity.

Work around the change by using `is_multiple_of` instead.

[breaking-change]