Refactor Struct class ref/set with cfunc env.

[take-cheeze]

• Add few tests related to it.
• Add arena index restore for large struct creating.

[matz]

Why do you want to use mrb_proc_new_cfunc_with_env()?
Because you can?

[take-cheeze]

Sorry for incomplete explanations.

(large Struct member getter is Struct member getter of member index 10 or more)

(About the algorithm and advantage.)
Struct member setting and large Struct member getting could be done with constant time with this patch.
Current Struct linear searches all members name to do above.
This should be effective larger the number of members is and how many the number of member assigning times is.

(About the memory and the disadvantage.)
mrb_proc_new_cfunc_with_env will require 1 REnv object and sizeof(mrb_value) * env_size for overhead.
So when applying this patch it will require (REnv × 1 + sizeof(mrb_value) × 1) × (total of Struct member setter and large Struct member getter) per each new Struct define.(Requires 1 env to store member index.)
In additional it needs memory for mruby-proc-ext mrbgem too.

I think constant time member assigning and getting worth the additional memory uses.
Though I don't know how much the mruby-proc-ext dependency worth the algorithm advantage.

[take-cheeze]

And I just found that member getter and setter can be shared with all Struct since what it does is all same like ref_func. Just method name would be different.
So memory overhead can be more small.

[take-cheeze]

A part of this code is merged so I'll close this and make the Struct with cfunc env with another pull request.
@matz Thank you for merging the test code and arena restore.