Task seq builder using bug

[peterfaria-lula]

Fixes: #97

When a type implements both IDisposable and IAsyncDisposable,
F# cannot determine which overload of TaskSeqBuilder.Using to use.

[peterfaria-lula]

Relates to #97

[abelbraaksma]

@peterfaria-lula I hope it's ok that I hijacked your branch ;). Working on a solution. Thanks for providing the failing code as an example!

[abelbraaksma]

Solution is now as follows:

• Added MediumPriority (no others needed) with AutoOpen.
• Removed IPriority1 and IPriority2 marker interfaces, as they aren't needed when splitting overloads over modules.
• Changed the default namespace for TaskSeqBuilder to be directly under FSharp.Control (this simplifies other auto-opens).
• Improved the tests such that we are sure the dispose methods are called.
• Ensure IAsyncDisposable is preferred if IDisposable and IAsyncDisposable are both present. This behavior is protected by a test.

Thanks @peterfaria-lula for writing the repro tests, it made it a breeze to analyze and solve.

This will be available in v0.3.0.