Converting a string to a slice of runes before ranging over it

You may want to loop over the runes in a string. Instead of converting
the string to a slice of runes and looping over that, you can loop
over the string itself. That is,

```
for _, r := range s {}
```

and

```
for _, r := range []rune(s) {}
```

will yield the same values. The first version, however, will be faster
and avoid unnecessary memory allocations.

Do note that if you are interested in the indices, ranging over a
string and over a slice of runes will yield different indices. The
first one yields byte offsets, while the second one yields indices in
the slice of runes.

