perf(api): allow to use an arena for return values

src/nvim/memory.h

@@ -6,6 +6,8 @@
 #include <stdint.h>  // for uint8_t
 #include <time.h>  // for time_t
 
+#include "nvim/macros.h"
+
 /// `malloc()` function signature
 typedef void *(*MemMalloc)(size_t);
 
@@ -37,6 +39,8 @@ extern MemRealloc mem_realloc;
 extern bool entered_free_all_mem;
 #endif
 
+EXTERN size_t arena_alloc_count INIT(=0);
+
 typedef struct consumed_blk {
   struct consumed_blk *prev;
 } *ArenaMem;