The `REnv` object is difficult to deal with, and it would be ideal if the user did not have to manipulate it directly.
In some previous situations, it was necessary to call `mrb_env_unshare()`, a non-API function, after `mrb_load_string()` or similar.
With this patch, it is no longer necessary for users to use `mrb_env_unshare()` directly, as it is now handled internally simply by using the `mrb_vm_ci_env_clear()` function.
Also, `mrb_vm_ci_env_set()` is demoted from the `MRB_API` function for the same reason.
ref. commit 1ab3da6f08
dearblue
38d5ed44e1