Add SingleThreadedSynchronizationContext as public API

[AArnott]

In this PR, I promote our existing SingleThreadedSynchronizationContext test class to a product class. Along the way, I added a bunch of tests focused on the class and fixed a few issues they found.

Since we still want to use WPF's SynchronizationContext when we can as part of testing, I kept the original class in the test project, but renamed it to avoid a name collision. That caused a lot of noisy diffs in the test project. The only interesting changes in the test project to review are to the sync context class itself and the tests for it.

Closes #323

[AArnott]

CC: @weltkante

[weltkante]

If I understand it correctly CreateCopy isn't supposed to reuse the instance because reference equality of SynchronizationContext is used to inline invocations in some code paths. This could cause inlining invocations on the wrong thread if SynchronizationContext is flown to a different thread via ExecutionContext. At least thats what I read out of the comments in the WPF SynchronizationContext (comment on the compat switch here)

[AArnott]

Thanks for the comment link. I didn't expect other code to use reference equality to mean they could skip using the SyncContext. Yikes. I'll fix this.

[weltkante]

Generally looks good for my usage, but I'm not sure about semantics of CreateCopy. When researching the WPF implementation I noticed they had originally returned this as well and later had to fix it. See comment above with link to reference source.