py/gc: Add "max new split" value in result of gc.mem_free().

Follow-up to 519c24dd when MICROPY_GC_SPLIT_HEAP_AUTO is enabled, based
on discussion at
https://github.com/orgs/micropython/discussions/12316#discussioncomment-6858007

gc.mem_free() is always a heuristic, but this makes it a more useful
heuristic for common use cases.
Signed-off-by: Angus Gratton <angus@redyak.com.au>

py/gc.c

@@ -701,6 +701,11 @@ void gc_info(gc_info_t *info) {
 
     info->used *= BYTES_PER_BLOCK;
     info->free *= BYTES_PER_BLOCK;
+
+    #if MICROPY_GC_SPLIT_HEAP_AUTO
+    info->max_new_split = gc_get_max_new_split();
+    #endif
+
     GC_EXIT();
 }
 
@@ -1159,7 +1164,7 @@ void gc_dump_info(const mp_print_t *print) {
     mp_printf(print, "GC: total: %u, used: %u, free: %u",
         (uint)info.total, (uint)info.used, (uint)info.free);
     #if MICROPY_GC_SPLIT_HEAP_AUTO
-    mp_printf(print, ", max new split: %u", (uint)gc_get_max_new_split());
+    mp_printf(print, ", max new split: %u", (uint)info.max_new_split);
     #endif
     mp_printf(print, "\n No. of 1-blocks: %u, 2-blocks: %u, max blk sz: %u, max free sz: %u\n",
         (uint)info.num_1block, (uint)info.num_2block, (uint)info.max_block, (uint)info.max_free);

py/gc.h

@@ -75,6 +75,9 @@ typedef struct _gc_info_t {
     size_t num_1block;
     size_t num_2block;
     size_t max_block;
+    #if MICROPY_GC_SPLIT_HEAP_AUTO
+    size_t max_new_split;
+    #endif
 } gc_info_t;
 
 void gc_info(gc_info_t *info);

py/modgc.c

@@ -64,7 +64,12 @@ MP_DEFINE_CONST_FUN_OBJ_0(gc_isenabled_obj, gc_isenabled);
 STATIC mp_obj_t gc_mem_free(void) {
     gc_info_t info;
     gc_info(&info);
+    #if MICROPY_GC_SPLIT_HEAP_AUTO
+    // Include max_new_split value here as a more useful heuristic
+    return MP_OBJ_NEW_SMALL_INT(info.free + info.max_new_split);
+    #else
     return MP_OBJ_NEW_SMALL_INT(info.free);
+    #endif
 }
 MP_DEFINE_CONST_FUN_OBJ_0(gc_mem_free_obj, gc_mem_free);