proposal: rename mrb_intern2 to mrb_intern

cf. #1512

Even I'm not sure now is the time to discuss this change (rather feeling somehow too early to change), I suggest the pull-request an API change in the name of intern function. As of now mruby has 4 kinds of intern functions `mrb_intern2`, `mrb_intern_cstr`, `mrb_intern_str`, and `mrb_intern`. The last one, because of a historical reason, is implemented as a bare wapper redirecting to `mrb_intern_cstr` for the backward compatibility. This patch is something simply do two things: removes the traditional `mrb_intern` function and replaces it with a same function as `mrb_intern2`. Changes in the commits may affect to very wide range. I do not have a strong opinion this proposal should be merged as soon as possible, but in fact I feel uncomfortable with the current asymmetric APIs.

Finally here I list up some possible pros/cons for the review
- Pros
    - clear meaning of API names
    - earlier is better (changing it early before the release is a good thing)
- Cons
    - breaks the backward-compatibility
    - breaks the compatibility with CRuby API (there is a function with the same style name in CRuby)