Feature Request: Substring from StringBuilder

Copy a sub string from StringBuilder to an external data structure such as Array

https://docs.oracle.com/javase/8/docs/api/java/lang/StringBuilder.html#getChars-int-int-char:A-int-

```
sb = sb.Append("a").Append("b").Append("c")
r := make([]rune, 2)
sb.Substring(0,2,r) -> "ab"
```