hand merge

 *  mrblib/init_mrblib.c
 *  mrblib/string.rb
 *  src/codegen.c
 *  src/dump.c
 *  src/encoding.h
 *  src/error.h
 *  src/gc.c
 *  src/load.c
 *  src/node.h
Merge branch 'iij-repair' of github.com:iij/mruby into iij-repair