Make binary operators take their arguments by value

- The following operator traits now take their arguments by value: `Add`, `Sub`, `Mul`, `Div`, `Rem`, `BitAnd`, `BitOr`, `BitXor`, `Shl`, `Shr`. This breaks all existing implementations.

- The binary operation `a OP b` now "desugars" to `OpTrait::op_method(a, b)` and consumes both arguments.

- `String` and `Vec` addition have been changed to reuse the LHS owned value, and to avoid internal cloning. Only the following asymmetric operations are available: `String + &str` and `Vec<T> + &[T]`, which are now a short-hand for the "append" operation.

[breaking-change]

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This passes `make check` locally. I haven't touch the unary operators in this PR, but converting them to by value should be very similar to this PR. I can work on them after this gets the thumbs up.

@nikomatsakis r? the compiler changes
@aturon r? the library changes. I think the only controversial bit is the semantic change of the `Vec`/`String` `Add` implementation.
cc #19148