fix typo (i->idx)
Merge pull request #2608 from mrbrdo/fix-typo

fix typo (i->idx)
remove header prototype of mrb_exc_print; close #2607
context proc may be cfunc; fix #2609
need addtional type check to avoid SEGV; ref #2609
Add Kernel.Array
Merge pull request #2611 from suzukaze/add-kernel.Array

Add Kernel.Array
refactoring C part of #2611
Refactor mrb_f_array func
Merge pull request #2612 from suzukaze/refactor-mrb_f_array

Refactor mrb_f_array func
instance_methods etc should not include undef'ed method names; based on a patch from @cremno; fix #2613
Add Kernel.String
Merge pull request #2616 from suzukaze/add-kernel.String

Add Kernel#String