Remove unused mrb_sprintf
fixed NaN boxing compilation

Use MRB_SET_VALUE instead of directly accessing members of mrb_value
which are different if NaN boxing is enabled.
Merge pull request #1059 from mattn/remove_needless_sprintf

Remove unused mrb_sprintf
Merge pull request #1060 from cremno/nan_boxing-mrb_bool_value-fails

NaN boxing: fix compilation