Refactor: Decouple Visitor and Stream for Renderers

[nibra]

Covers PR#142.

Because the PSR-7 HTTP middleware requires the response body to be a message stream, the renderers were built to not only implement the visitor pattern, but also the PSR-7 message stream interface.

For layouts and templates, streams are not easy to handle, especially, when things need to be inserted in certain positions. Whenever the buffer content changes, the cursor position needs to be re-calculated. Even for appending it is needed to move the pointer to the end of the buffer first.

That is way too complicated and prune to errors.

This PR separates the stream from the renderers. A renderer manages its own string buffer, if needed,
After rendering, the buffer is wrapped into a PSR-7 compatible stream and attached as body to the response, as that is the only place, where the output has to be a stream.

ContentTypeVisitorTrait is introduced with this PR to simplify the handling of visitor methods, especially in case they are very similar. Additionally, renderers using that trait are much more robust against changes in the ContentTypeVisitorInterface, as they automatically provide a fallback for new content types.