This is the minimum version that compiles/works, people can still use a newer Go on their programs. I usually like to represent the min and then more people can use the lib (maybe someone has to use 1.20..for some reason, etc).

I like this to avoid naughty ordering issues when testing + avoid the test running cache (which I dont like).

Also renamed from "muxer" to "mux", it is smaller and more in line with Go naming (not a verb).
This is a breaking change but that is OK IMHO.
Now the mux code is super simple and brings us pretty close to Limbo behavior of being able to iterate on slices of channels (we only need the explicit function call, but pretty close :-)).
Thinking on using this when implementing a very simple/specific load test.

And remove CI/build stuff.