Leaf nodes could be strings.

[Here](https://github.com/miku/zek/blob/83570a9892d9b053c08259a27448138e7a4d87ca/fixtures/r.go#L1489-L1491) we have:

```
KASS struct {
    Text string `xml:",chardata"`
} `xml:"KASS"`
```

which could be expressed as well as:

```
KASS string `xml:"KASS"`
```

Shorter, but would break uniformity (.Text for chardata everywhere).
