Add carry/borrow to Fallible<MutableDataInt.Body> #148

The MutableDataInt<Element>.Body model has many operation pairs similar to this:

extension MutableDataInt.Body {
    consuming func increment(..............) -> Fallible<Self>
    consuming func incrementSubSequence(...) -> Fallible<Self>
}

The *SubSequence operations represent some core algorithm that stops at the end of the given arguments. The non-*SubSequence carries or borrows the error indicator after that point. What if I instead add a carry() and borrow() to Fallible<MutableDataInt.Body>? I believe those two methods would make every non-*SubSequence variant obsolete.