Amend arith-oflo with `wrapping_neg` in the `WrappingOps` trait

[pnkfelix]

The tracking issue for RFC #560 , namely rust-lang/rust#22020, mentions implementing a wrapping_neg method, but no such method is part of the trait as defined in the RFC.

(Presumably just an oversight.)

[pnkfelix]

(or oh, maybe that line item was a relatively recent addition...)

((also, @glaebhoerl already suggested potentially removing unary negation on unsigned integers as part of this, shown here: #560 (comment) ... it certainly feels a little strange to keep it with all of these other checks going in to ensure that you are doing "good" arithmetic.))

[petrochenkov]

wrapping_neg is implemented now