SE-0300: Continuations for interfacing async tasks with synchronous code

[benrimmington]

@ktoso

So yeah, either top level or nested in Task, though I don't remember the specific reason we ended up prefering the top-level one rather the ones defined in Task

In the pitch thread, I suggested the existing Result enum, so that we can use:

• Result<T, Never> instead of separate non-throwing APIs.
• resume(.success(value)) instead of resume(returning: value).
• resume(.failure(error)) instead of resume(throwing: error).

Are the with*Continuation methods misnamed? The continuation is allowed to escape the given closure, unlike other with* methods in the standard library.