fix bugs on Heredocument
- heredoc in array literal
- heredoc in args
- heredoc in expression expand
eliminate use of traditional intern API (mrb_intern()) completely
fix a bug introduced in prev commit
Merge pull request #1512 from wasabiz/eliminate-mrb-intern

eliminate use of traditional intern API (mrb_intern()) completely
change type of mrb->c->status from uint8_t to enum mrb_fiber_state; #1511
Merge pull request #1509 from FUKUZAWA-Tadashi/heredoc-bugfix

fix bugs on Heredocument
fix build error when ENABLE_DEBUG is defined
Merge pull request #1515 from mynz/master

fix build error when ENABLE_DEBUG is defined