Track the number of live multiplexers and report it in the error dump if non-trivial.

Inspired by #3029, although I must emphasize that I do not have sufficient evidence to know whether this is a related factor... but the fact that it is even a question in my mind: makes it seem worthwhile.

Impact: minimal; adds an interlocked increment to muxer create, dispose and finalize; we already have a finalizer (nixed in dispose), so: this isn't additional pain.

Secondary change here is re an updated analyzer: https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/breaking-changes/compiler%20breaking%20changes%20-%20dotnet%2010#warn-for-redundant-pattern-in-simple-or-patterns - I installed SDK 11 preview 2 locally today, and now I get disagreements with the CI server on when this applies; easiest thing is to side-step it entirely!