simplify the exchange allocator

* stop using an atomic counter, this has a significant cost and
  valgrind will already catch these leaks
* remove the extra layer of function calls
* remove the assert of non-null in free, freeing null is well defined
  but throwing a failure from free will not be
* stop initializing the `prev`/`next` pointers
* abort on out-of-memory, failing won't necessarily work

src/rt/rust_kernel.cpp

@@ -211,7 +211,6 @@ rust_kernel::run() {
     assert(osmain_driver != NULL);
     osmain_driver->start_main_loop();
     sched_reaper.join();
-    rust_check_exchange_count_on_exit();
     return rval;
 }