Move to standard Request and Response objects

[evert]

The Request and Response objects are becoming standard everywhere. Deno uses them, and so does Bun. When Curveball was created they were still up and coming, so it ships its own Request and Response types that look a lot like Koa's.

I think for Curveball to stay relevant, it should probably adopt these interfaces. The biggest issue with this is that this is a drastic BC break. I also don't even fully know if it's possible to do this, given that Curveball assumes that the Response object is mutable and potentially changed by middlewares.