Remove dangerous mrb_gc_arena_restore

```mrb_gc_arena_restore``` cancels a GC protection from ``` value ```,
 and so ``` value ``` can be removed in the C function.

```bash
bin/mirb
mirb - Embeddable Interactive Ruby Shell

This is a very early version, please test and report errors.
Thanks :)

> loop { JSON.stringify({a: "\n"}) }
Segmentation fault: 11
```