Add a huge hack to allow the test runner to continue if a single task leaks

This is just until unwinding works. Adds a flag to the runtime to turn
the memory leak checks on task destruction into warnings instead of fatal
errors. I am so sorry.

Issue #428

src/rt/rust_builtin.cpp

@@ -459,6 +459,13 @@ debug_opaque(rust_task *task, type_desc *t, uint8_t *front)
     }
 }
 
+extern "C" CDECL void
+hack_allow_leaks(rust_task *task)
+{
+    LOG(task, stdlib, "hack_allow_leaks");
+    task->local_region.hack_allow_leaks();
+}
+
 struct rust_box {
     RUST_REFCOUNTED(rust_box)