[javac plugin] Add compile-time checks for unsafe or incorrect coroutine usage

[SamCarlberg]

CoroutineYieldInLoopDetector

This checks for while loops where coroutines are in scope but without calling a blocking method on at least one of those coroutines:

drivetrain.run(theCoroutine -> {
  while (drivetrain.getDistance() < 10) { // ERROR: "Missing call to `theCoroutine.yield()` inside loop"
    drivetrain.setSpeed(1);
  }
});

Note that, because we assume most looping constructs in commands will use while loops, we don't check for-loops, for-each loops, or do-while loops.

This check can be disabled with @SuppressWarnings("CoroutineYieldInLoop")

CodeAfterCoroutineParkDetector

Essentially acts like the Java compiler's check for code after a while (true) loop, but for coroutine.park():

drivetrain.run(theCoroutine -> {
  drivetrain.setSpeed(1.0);
  theCoroutine.park();
  drivetrain.setSpeed(0.0); // ERROR: "Unreachable statement: `theCoroutine.park()` will never exit"
});

This check can be disabled with @SuppressWarnings("CodeAfterCoroutinePark")

IncorrectCoroutineUseDetector

Checks for usage of captured (outer) coroutine parameters and assignments to fields.

drivetrain.run(outer -> {
  outer.await(arm.run(inner -> {
    outer.yield(); // ERROR: "Coroutine `outer` may not be in scope. Consider using `inner`"
  }))
});

This check can be disabled with @SuppressWarnings("CoroutineMayNotBeInScope")

private Coroutine coroutineField;
drivetrain.run(co -> coroutineField = co); // ERROR: "Captured coroutines may not be stored in fields"

This check can be disabled with @SuppressWarnings("CoroutineCapture")