use mrb_str_cat_lit() intead of mrb_str_cat
Refactor tests in kernel.rb
make mrb_str_dump a bit faster
add mruby-strip tool to strip irep's debug info
Merge pull request #1757 from cubicdaiya/issues/use_mrb_str_cat_lit

use mrb_str_cat_lit() intead of mrb_str_cat
Merge pull request #1759 from suzukaze/fix-test

Refactor tests in kernel.rb
Merge pull request #1758 from ksss/array-out-while

move check condition to outside in while block
Merge pull request #1760 from cubicdaiya/issues/make_mrb_str_dump_a_bit_faster

make mrb_str_dump a bit faster
Merge pull request #1761 from take-cheeze/mruby-strip

Add mruby-strip tool to strip irep's debug info.
remove break altogether in ci loop
fix SEGV bug for mrb_str_new

mrb_str_new causes seg-fault when 3rd argument is negative.
Merge pull request #1762 from cubicdaiya/issues/mrb_str_new_bug

fix SEGV bug for mrb_str_new
fix test for Array#[]