Add Proc#arity

Issue https://github.com/mruby/mruby/issues/1068

```bash
$ mirb
mirb - Embeddable Interactive Ruby Shell

This is a very early version, please test and report errors.
Thanks :)

> Proc.new {|a, b|}.arity
 => 2
> Proc.new {|a, *b|}.arity
 => -2
> Proc.new {|a, b = 0, c|}.arity
 => 2
>
```