- `Task` and `Task<T>` are classified as task types despite not
  specifying builder types
- Interfaces are allowed to be task types
- Enums and methods are prohibited from being decorated with
  AsyncMethodBuilderAttribute
- Task types must not be generic in more than one type parameter
  (including in terms of containing types)

Discussion required for all of this, but if merged, would
fix dotnet#854, dotnet#856, dotnet#858 and dotnet#859.

This wording follows the wording in 15.15.2 for task builders.