Improve ExposedThing - Binding comunication

PR #543 introduced a new method for handling content between the core module and the binding level. We should restructure `ExposedThing` following the same approach for consistency and improving the overall handling of streams also at the exposed side.

A couple of open questions:
- How to handle the `response` parameter forms?
- A protocol binding must also provide the form where the request was issued, how can it correctly choose it? currently, my heuristic is to find the first form with the right protocol scheme and path. But we have other parameters to check like the contentType.
- An interaction handler expects also UriVariables filled in, how we can populate them?
- Should we keep the old ConsumedThing methods?

Related to #525