Implement Kernel.fail and Kernel#fail .
OP_ENTER fix variable assignment
Move Kernel#fail definition to mruby-kernel-ext mrbgem.
Add mruby-kernel-ext to default gembox.
Merge pull request #2205 from ksss/proc-m2

OP_ENTER fix variable assignment
Merge pull request #2204 from take-cheeze/kernel_fail

Implement Kernel.fail and Kernel#fail .