Use an array instead of a hash table for id_str

Ruby spends a non-trivial amount of time in `rb_id2str` when booting the GitHub Rails app:

![screen shot 2013-12-01 at 10 17 02 pm](https://f.cloud.github.com/assets/179065/1650278/9bdfef58-5a7a-11e3-8637-562ad91605e1.png)

This pull request changes `global_symbols.id_str` to be an array instead of a hash table. Because ID values are sequentially increasing integers, using a hashtable for `id_str` is unnecessary and slow.

This improves the boot time of our app by about 150-200ms.

Before:

```
        6.65 real         5.40 user         1.23 sys
        6.76 real         5.47 user         1.25 sys
        6.58 real         5.35 user         1.20 sys
        6.62 real         5.39 user         1.20 sys
        6.69 real         5.44 user         1.21 sys
```

After:

```
        6.41 real         5.19 user         1.19 sys
        6.45 real         5.22 user         1.20 sys
        6.45 real         5.21 user         1.20 sys
        6.46 real         5.22 user         1.20 sys
        6.43 real         5.20 user         1.20 sys
```

cc @ko1