Re-order enum variants of `AssignmentOperator`, `BinaryOperator` and `UnaryOperator`.
* `Exponential` moved to after `Remainder` (so with the rest of the arithmetic operators).
* `Shift*` operators follow arithmetic operators.
* `AssignmentOperator::Bitwise*` ops moved to before `Logical*` ops (so all ops which correspond to `BinaryOperator`s are together).
* `*Or` always before `*And`.
* Plus/Addition always before Minus/Subtraction.
The purpose is to make the various methods on these types maximally efficient:
1. Group together variants so that `AssignmentOperator::is_*` methods can be executed with the minimum number of operations (essentially `variant - min <= max`).
2. Align the variants of `AssignmentOperator` and `BinaryOperator` so that conversion methods added in #7350 become very cheap too (essentially `if variant - min <= max { Some(variant + offset) } else { None }`).
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