allof.c: mrb_default_allocf() definition in the separate file

So that the user can define their own version of mrb_default_alloc
function to override memory allocation of mruby.

src/allocf.c

@@ -0,0 +1,30 @@
+/*
+** allocf.c - default memory allocation function
+**
+** See Copyright Notice in mruby.h
+*/
+
+#include <stdlib.h>
+#include "mruby.h"
+
+/* This function serves as the default memory allocation function and accepts four arguments:
+ *
+ * - `mrb`: An instance of `mrb_state`. It's important to note that for the initial allocation (used to allocate the `mrb_state` itself), `mrb` is set to NULL.
+ * - `p`: The previous pointer to the memory region. For memory allocation, this parameter is NULL.
+ * - `size`: The new size of the memory region to be returned.
+ * - `ud`: User data, represented as a `void*`, which is passed to the `mrb_state`.
+ */
+
+void*
+mrb_default_allocf(mrb_state *mrb, void *p, size_t size, void *ud)
+{
+  if (size == 0) {
+    /* `free(NULL)` should be no-op */
+    free(p);
+    return NULL;
+  }
+  else {
+    /* `ralloc(NULL, size)` works as `malloc(size)` */
+    return realloc(p, size);
+  }
+}

src/state.c

@@ -58,18 +58,6 @@ mrb_open_core(mrb_allocf f, void *ud)
   return mrb;
 }
 
-void*
-mrb_default_allocf(mrb_state *mrb, void *p, size_t size, void *ud)
-{
-  if (size == 0) {
-    free(p);
-    return NULL;
-  }
-  else {
-    return realloc(p, size);
-  }
-}
-
 MRB_API mrb_state*
 mrb_open(void)
 {