REnv macros.

Since some usage of `REnv` is illegible I would like to have following macros:
* `MRB_ENV_STACK_LEN(env)` to retrieve stack length.
* `MRB_ENV_UNSHAR_STACK(env)` to make GC free stack in REnv free.
* `MRB_ENV_STACK_SHARED_P(env)` to check whether REnv's stack is shared.
  * By default this returns true. When it's unshared with `MEB_ENV_UNSHARE_STACK` it will return false.