inherit `#[align]` from trait method prototypes
````@workingjubilee```` this seems straightforward enough. Now that we're planning to make `-Cmin-function-alignment` a target modifier, I don't think there are any cross-crate complications here?
````@Jules-Bertholet```` is this the behavior you had in mind? In particular the inheritance of the attribute of a default impl is maybe a bit unintuitive at first? (but I think it's ok if that behavior is explicitly documented).
r? ghost