3.0-era plans

[StephenCleary]

Expected changes:

• ~Change SingleDisposable.TryUpdateContext semantics so that if it is called during disposal, it will block on that disposal before disposing the new child.~ Done as part of 2.1.0 (considering this a bug fix even though it does change semantics)
• Move SingleDisposable / SingleNonblockingDisposable into a different namespace so advanced users can "opt in".

Possibly before 3.0-era:

• ~Add AddRange for the collection. => Moved to #11~ Not planning to do this.
• ~Add a ref-counted disposable. => Moved to #12~ Done as of 2.3.0.
• ~Consider adding a simple "FixDisposable" that only wraps a single underlying disposable.~ Not planning to do this.

[RKayCH]

(y) me likes! Thank you very much for all your efforts!

[StephenCleary]

Idea:

• ~Define a simple Disposable and AsyncDisposable for the 95% use case so people don't have to remember AnonymousDisposable / AnonymousAsyncDisposable.~ Done as of 2.2.0.

[StephenCleary]

• Deprecate AnonymousDisposable / AnonymousAsyncDisposable.
• Change Single usage from inheritance to composition.

[StephenCleary]

All changes above are in v3.0.0-pre01.

[StephenCleary]

These improvements are in 2.1.0-2.3.0, and the backwards-incompatible changes are only usage changes that don't provide new functionality, so I'm closing this for now (and staying on 2.x for the foreseeable future).