Make unary operators take their arguments by value

- The following operator traits now take their argument by value: `Neg`, `Not`. This breaks all existing implementations of these traits.

- The unary operation `OP a` now "desugars" to `OpTrait::op_method(a)` and consumes its argument.

[breaking-change]

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r? @nikomatsakis This PR is very similar to the binops-by-value PR
cc @aturon