mruby-struct: internal linkage for all functions
mruby-struct: use correct data types
mruby-struct: remove unused functions
Release `mrb->atexit_stack` on `mrb_close`.
mruby-struct: implement `mrb_is_(local|const)_id`
Merge pull request #2364 from take-cheeze/free_atexit_stack

Release `mrb->atexit_stack` on `mrb_close`.
Merge pull request #2365 from cremno/mruby-struct-function-changes

various mruby-struct function changes
reduce calling mrb_sym2name_len() from struct.c; ref #2365