Add AsyncMethodBuilderAttribute to async monads (EitherAsync, OptionAsync, etc)

[fjmorel]

Refactoring methods to return EitherAsync<L, R> rather than Task<Either<L, R>> can be a challenge sometimes since you can't use async/await. This is the error if you try to use async with EitherAsync as the return type (emphasis mine):

The return type of an 'async' method must be a 'void', 'Task', 'Task', a task-like type, 'IAsyncEnumerable', or 'IAsyncEnumerator'

Based on https://devblogs.microsoft.com/premier-developer/extending-the-async-methods-in-c/#task-like-types I believe it would be possible to update OptionAsync, EitherAsync, etc to be usable as return types of async methods.

I noticed recently that I can now await these types (thank you!), so this would be an extra thing that makes them easier to use.

edit: more discussion here: #960

[fjmorel]

If this is desirable, I'd be willing to try implementing it myself.

[louthy]

This already exists for OptionAsync, but not for EitherAsync. This approach doesn't work for types with multiple generic arguments, because the C# team's hack to make async work through the god-awful attributes and GetAwaiter will only look for types with a single generic parameter.

[fjmorel]

Oh, I didn't know that. 😢

Do you happen to know if the compiler team is aware of that? I could file or upvote an issue in dotnet/roslyn

[fjmorel]

Found it, which you already commented on: dotnet/csharplang#3723