Print backtrace of crashed test in verbose mode.
Merge pull request #2480 from take-cheeze/test_backtrace

Print backtrace of crashed test in verbose mode.
Move word boxing structs to "mruby/boxing_word.h".
Merge pull request #2481 from take-cheeze/move_word_box_struct

Move word boxing structs to "mruby/boxing_word.h".
since #1441, NaN boxing with 64bit pointer works; close #2476
exception in instance_eval should not exit mrb_run; fix #2483
Merge branch 'master' of https://github.com/mruby/mruby

Conflicts:
	include/mruby/object.h