`Integer::next_multiple_of` unexpectedly fails on `MIN` and `-1`

[jhpratt]

i8::MIN.next_multiple_of(-1) should return -128 (i8::MIN), but instead panics. The fix for this is to add if self == -1 { return self; } at the top of the method.

I have not checked, but it is certainly worth checking whether a similar situation applies with prev_multiple_of.

This is the only issue with the current implementation of next_multiple_of. It otherwise works as expected.