Improve `StringBuf.add` to increase performance

My problem lies with this:

``` haxe
class StringBuf {
public inline function add(x :T):Void;
}
```

Because of this signature, anything added to a `StringBuf` gets wrapped in `Std.string`, which is true for `String` itself.

To milk the irony of the situation: there is no fast way to add a `String` to a `StringBuf`.

Therefore I propose this change:

``` haxe
class StringBuf {
public inline function add(x:StringBufFragment):Void;
}

private abstract StringBufFragment(String) from String to String {
// possibly add fast paths here for @:from Int/Float/Bool
@:from static inline function ofAny(a:A):StringBufFragment return Std.string(a);
}
```

Would such a change be ok?