Iterator arithmetic

I suggest we use the standard functions `std::next`, `std::prev`, `std::advance` and so on instead of operators like `++`, `--`, and `+=` to perform iterator arithmetic. In particular, `std::next` and `std::prev` are guaranteed to work on _rvalues_ while the operators may not:

``` cpp
++c.begin();          // may not compile
std::next(c.begin()); // surely compiles
```
