Add a way to convert Bundle to a MagicString

[RReverser]

Right now, Bundle is the only way to concatenate couple of MagicStrings together. However, if you use it, you can't use all the other methods of MagicString and there's no way to convert from Bundle to a "flat" MagicString either.

It would be great to provide easier options to concatenate some MagicStrings together, while allowing to do further processing on the result.