Safe handling of physical actions in single-threaded C runtime

[lhstrh]

Prior to these changes, it was not possible to asynchronously access the event queue from an interrupt service routine on bare-iron platforms. This PR -- currently still a work in progress -- should enable the use of physical actions on embedded platforms.
Known issues:

• the compiler currently forces the the use of the threaded runtime if there are physical actions; this behavior needs to be changed, but it's not clear how exactly. Not all embedded platforms are single-threaded.
• ...

[petervdonovan]

The tracing tests are failing on Windows for both Python and C with ERROR_ACCESS_DENIED, which seems to just be the same as a segmentation fault, and I cannot reproduce the error locally, even on Windows.

[lhstrh]

That is rather bizarre... 😔

[lhstrh]

It sounds like a race condition. I believe that if tracing is enabled the threaded runtime is always used...

[edwardalee]

See lf-lang/reactor-c#106 (review)

[erlingrj]

This is ready for merge. Arduino is still supported as a target platform. I think you removed the support in reactor-c, so should we remove it here as well?

[lhstrh]

Not sure about Arduino. It kind of depends on what @arengarajan99 is up to, but I haven't heard from him in a while.

[lhstrh]

Looks great, @erlingrj! Thanks for much for whipping this into shape.