Add abs for all signed ints

[chipshort]

Pulled out of #1807

[webmaster128]

Looks good. Could you add a few tests for those?

[webmaster128]

Looks good. We can also test zero:

assert_eq!(Int512::zero().abs(), Int512::zero());

One other thought: Is there any standard way to do a sigened -> unsigned conversion via abs? Would you do abs_diff with zero?

[chipshort]

Not via abs, but there is unsigned_abs. I think it would be functionally equivalent to abs_diff with zero (probably saves a few instructions)

[webmaster128]

but there is unsigned_abs.

Oh, nice. This is even available for primitive types. Can we add this too here? It's nice as it cannot panic.