Explcit impure function attribute

[IchorDev]

It might be a bit of a hassle to update, but since `pure` is a 
keyword right now it'd be easy to just find-and-replace it with 
`@pure`, so I'm all for `@pure`, `@impure`, and no more `pure` 
keyword.

On Wednesday, 26 June 2024 at 17:51:46 UTC, Richard (Rikki) 
Andrew Cattermole wrote:
> there is significant desire by core developers for it.

So Phobos 3 will use SumType exceptions out of the gate?