mrb_sym2name_len become faster.

mrb_sym2name_len used linear search.

this image is profiling result of my software.

![profile_mruby](https://cloud.githubusercontent.com/assets/553189/4263042/67d9df74-3bc6-11e4-8158-1eda69e0a1c4.png)

I change linear search to searching by hash.