int::abs should return an unsigned int

[catamorphism]

Currently, int::abs has a return type that's the same as its arg type, and writing int::abs(x) as uint looks very silly. I understand that it's not totally straightforward to write what we really want (basically, we want a type function that relates an int type of a given size to a uint type of a given size, and AFAICT we can't do that), but it would still be good. Maybe there's a clever workaround I'm not seeing...

[nikomatsakis]

I am not sure if this makes sense. I imagine a common use of abs is in some formulas where the result will still be combined with other ints, not uints.

I imagine it is actually very unusual to have code where you have an int which could be negative and you need to supply that int to a uint context such that if the int is negative you want the absolute value. I could imagine something like int::to_uint() which asserted that the int was indeed positive... currently we just do i as uint which is of course unsafe in this regard.

[catamorphism]

I'll take your word for it. Closing.