fix fiber GC problem
mruby error messages should be directed to stderr
Add MRB_WORD_BOXING mode (represent mrb_value as a word)
Merge pull request #1266 from kimushu/feature/word-boxing

Add MRB_WORD_BOXING mode (represent mrb_value as a word)
Merge branch 'master' of github.com:mruby/mruby
add Fiber#alive?
Merge pull request #1267 from skandhas/pr-add-Fiber-alive_p

add Fiber#alive?