std.string.remove(Front|Back|First|Last|FirstN,LastN)

I found these function to be quite common when processing strings.

These functions are not hard to implement, but why bother the developer with all the length handling and such.


I changed the order because:

```
someString.eraseFristN(a_lot_of_nested_calls_and_stuff, someNumber); 
```

may just be hard to read, but I mind changing it again