proposal: bytes, strings: add Truncate

I find myself implementing this function pretty often and I thought it would be decent addition to the standard library.

``` go
// Truncate returns the first n runes of s.
func Truncate(s string, n int) string {
	if len(s) <= n {
		return s
	}
	for i := range s {
		if n == 0 {
			return s[:i]
		}
		n--
	}
	return s
}
```


