Use `mrb_bool` as return type of `mrb_const_defined_at`.
Set `$mrbtest_verbose` flag in `mrb_init_test_driver`.
Merge pull request #2464 from take-cheeze/inherit_verbose_flag

Inherit `$mrbtest_verbose` flag in core tests.
Merge pull request #2471 from take-cheeze/const_defined_at_ret

Use `mrb_bool` as return type of `mrb_const_defined_at`.
backtrace supression condition was wrong; fix #2472
fiber stack allocation size should be bigger than irep->nregs
Implement string compiling `instance_eval` method.
remove unused local variables; ref #2473
git push origin masterMerge branch 'take-cheeze-instance_eval'
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
Merge remote-tracking branch 'upstream/master' into ext