How to get n-th symbol of utf-8 string

[DavideGalilei]

Possible duplicate of <https://forum.nim-lang.org/t/1067>, however this is what 
I came up with:
    
    
    import unicode
    
    let s = "Привет!"
    echo s.runeStrAtPos(1)
    
    
    Run

Which outputs "р" <https://play.nim-lang.org/#ix=3BQU>

Warning: as [stated in 
documentation](https://nim-lang.org/docs/unicode.html#runeStrAtPos%2Cstring%2CNatural),
 this proc may lead to performance issues, so someone might come up with a 
better and more efficient solution.