Use boolean macro in kernel.c
break in blocks cannot cross C function boundaries; fix #2298
direct invocation of module_eval; ref #2298
Check mrbc_context is null
direct invocation of instance_eval
Merge pull request #2301 from mimaki/fix-load-mrb-segv

mrb_load_irep_file() caused SEGV
Merge pull request #2286 from suzukaze/refactor-kernel

Use boolean macro in kernel.c