cleanup warnings in hash.c

```
mruby/src/hash.c:159:1: warning: ‘mrb_hash_dup`defined but not used [-Wunused-function]
 mrb_hash_dup(mrb_state *mrb, mrb_value hash)
     ^
```

this pr defines Hash#dup to use `mrb_hash_dup` in hash.c, however it seems that Hash#dup is not in the ISO standard yet.