Adresses pr #165 and issue #164 by @colesbury:

On Mac OS, the thread-local _mi_default_heap may get reset before
_mi_thread_done is called, leaking the default heap on non-main threads.

Now the current default heap is also stored in mi_pthread_key (or mi_fls_key
on Windows). The _mi_thread_done function is called with this value.

include/mimalloc-internal.h

@@ -101,6 +101,7 @@ uint8_t    _mi_bsr(uintptr_t x);                // bit-scan-right, used on BSD i
 void       _mi_heap_destroy_pages(mi_heap_t* heap);
 void       _mi_heap_collect_abandon(mi_heap_t* heap);
 uintptr_t  _mi_heap_random(mi_heap_t* heap);
+void       _mi_heap_set_default_direct(mi_heap_t* heap);
 
 // "stats.c"
 void       _mi_stats_done(mi_stats_t* stats);