Why does 'string' and 'len' use 'int' rather than 'uint' for length?

[Hlaaftana]

Mostly because Nim did not originally have unsigned integers. Beyond that, 
constant type conversions are annoying and it's not like there's a problem of 
`int` not allowing high enough values. Even if it did, imagine if it 
overflowed, would you prefer it has the valid length 0 or an invalid negative 
length?