Add mrb_alloca again; ref #4470

This time, the allocated memory comes from the string object, which is
referenced from GC arena. The memory region will be reclaimed when the C
function called from VM is terminated, or the GC arena is restored.

include/mruby.h

@@ -1246,6 +1246,8 @@ MRB_API void mrb_pool_close(struct mrb_pool*);
 MRB_API void* mrb_pool_alloc(struct mrb_pool*, size_t);
 MRB_API void* mrb_pool_realloc(struct mrb_pool*, void*, size_t oldlen, size_t newlen);
 MRB_API mrb_bool mrb_pool_can_realloc(struct mrb_pool*, void*, size_t);
+/* temporary memory allocation, only effective while GC arena is kept */
+MRB_API void* mrb_alloca(mrb_state *mrb, size_t);
 
 MRB_API void mrb_state_atexit(mrb_state *mrb, mrb_atexit_func func);
 

src/gc.c

@@ -277,6 +277,13 @@ mrb_free(mrb_state *mrb, void *p)
   (mrb->allocf)(mrb, p, 0, mrb->allocf_ud);
 }
 
+MRB_API void*
+mrb_alloca(mrb_state *mrb, size_t size)
+{
+  mrb_value str = mrb_str_new(mrb, NULL, size);
+  return RSTRING_PTR(str);
+}
+
 static mrb_bool
 heap_p(mrb_gc *gc, struct RBasic *object)
 {