Convert stream to boolean explicitly

<del>
(U16TrivialInputStream::operator bool): Replace with istream::good call
(U16TrivialInputStream::operator!): Replace with istream::good call

`operator bool` is libstdc++ extension. So libc++ doesn't have this feature.
</del>

__edit__:
Changed issue title to "Convert stream to boolean explicitly"
[See comment](https://github.com/esrille/escort/pull/27#issuecomment-13386762)