mrb_hash_ht_hash_func use mrb_obj_id() instead of mrb_sym2name_len()

`hash[:symbol]` is usecase of commonly-used.

But mrb_sym2name_len have cost that search symbol from n2s table.
And `Symbol.object_id` is never change.

I propose to use mrb_obj_id() when hash search symbol key.

Simplicity benchmark https://gist.github.com/ksss/10088148