Auto merge of #423 - Mingun:translation, r=nical
Auto merge of #423 - Mingun:translation, r=nical

Some refactoring, *Assign operators and box transform methods for Translations

As a continuation of [our discourse](https://github.com/servo/euclid/pull/411#discussion_r395735133) about by value vs by reference: added translation methods for Boxes takes its by reference, but as [godbolt](https://rust.godbolt.org/z/Kgqdjd) shows, even without forced `#[inline]` directive compiler smart enough to inline such small function and the code the same in both cases. Therefore, I'm still in favour of taking everything by value in methods that almost only change unit. For now I'm, however, use reference as in surrounding code.

servo/rust-geom