After chatting to Facebook's Carl Shapiro here at the core dev sprint, I'm going to start exploring a hopefully more robust white-listing based approach where we check for pending calls in the following cases:

* the instruction pointer either hasn't changed or has gone backwards and the instruction isn't a YIELD_FROM (the "return to the start of a loop" case)
* the next instruction is RETURN_VALUE (the function postamble case)

For now, I'm going to *skip* checking for additional pending calls when exiting a frame via exception, based on the theory that if we're already handling an exception, it's OK to wait until something catches it before checking for potential additional exception sources.