auto merge of #19899 : japaric/rust/unops-by-value, r=nikomatsakis

- 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]

---

r? @nikomatsakis This PR is very similar to the binops-by-value PR
cc @aturon

src/test/run-pass/operator-overloading.rs

@@ -31,13 +31,13 @@ impl ops::Sub<Point,Point> for Point {
 }
 
 impl ops::Neg<Point> for Point {
-    fn neg(&self) -> Point {
+    fn neg(self) -> Point {
         Point {x: -self.x, y: -self.y}
     }
 }
 
 impl ops::Not<Point> for Point {
-    fn not(&self) -> Point {
+    fn not(self) -> Point {
         Point {x: !self.x, y: !self.y }
     }
 }