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

[Araq]

Bugs like `for (auto i = 0; i < s.length() - 3; ++i)` are everywhere in C++ and 
nowhere in C#, that's why. Using `uint` for "cannot be negative" is just wrong 
-- unsigned "integers" wrap around and are more like bitvectors than they are 
numbers.