Use MRB_ENV_STACK_LEN instead of accessing `flags` directly to get REnv's stack length.
Add macro MRB_ENV_UNSHARE_STACK instead of assigning -1 directly.
Use MRB_ENV_STACK_SHARED_P to check `cioff` field of REnv.
Merge pull request #1986 from take-cheeze/proc_macro

REnv macros.