need to call write_barrier on every fiber context switch
Check mrbgem.rake instead of directory to ignore empty directories in full-core.
Merge pull request #2475 from take-cheeze/full_core_list

Check mrbgem.rake instead of directory to ignore empty directories in full-core gembox.
avoid infinite recursion when error was happened in #inspect; fix #2474
Merge branch 'master' of github.com:mruby/mruby