Character position setter / rewind #122

I'm implementing look-behind in a parser library based on strscan; currently the only way to rewind the character position is this:

            pos = @str.charpos - nchars
            @str.reset
            @str.getch while @str.charpos < pos

which is not the most efficient it could be, with per-character roundtrips to the native extension and back to Ruby… Would be a lot nicer to have a setter for the character position and not just the byte one.