Use __morestack to detect stack overflow

This commit resumes management of the stack boundaries and limits when switching
between tasks. This additionally leverages the __morestack function to run code
on "stack overflow". The current behavior is to abort the process, but this is
probably not the best behavior in the long term (for deails, see the comment I
wrote up in the stack exhaustion routine).

src/rt/rust_test_helpers.cpp

@@ -11,7 +11,6 @@
 // Helper functions used only in tests
 
 #include "rust_util.h"
-#include "sync/rust_thread.h"
 #include "sync/lock_and_signal.h"
 
 // These functions are used in the unit tests for C ABI calls.