make mrb_str_new_cstr DRY

Using str_new eliminates duplicated codes in mrb_str_new_cstr.
Merge pull request #1292 from cubicdaiya/dry_mrb_str_new_cstr

make mrb_str_new_cstr DRY
Cleanup mrb_str_resize().
Merge pull request #1293 from monaka/pr-cleanup-mrb_str_resize

Cleanup mrb_str_resize().