initial api for heaps restricted to a certain arena

src/init.c

@@ -109,6 +109,7 @@ mi_decl_cache_align const mi_heap_t _mi_heap_empty = {
   MI_ATOMIC_VAR_INIT(NULL),
   0,                // tid
   0,                // cookie
+  0,                // arena id
   { 0, 0 },         // keys
   { {0}, {0}, 0 },
   0,                // page count
@@ -149,6 +150,7 @@ mi_heap_t _mi_heap_main = {
   MI_ATOMIC_VAR_INIT(NULL),
   0,                // thread id
   0,                // initial cookie
+  0,                // arena id
   { 0, 0 },         // the key of the main heap can be fixed (unlike page keys that need to be secure!)
   { {0x846ca68b}, {0}, 0 },  // random
   0,                // page count