Support automatic forward-paging of entity collections

[deviantintegral]

As a developer using the drupal.org API, it's common to have to load multiple pages of results. This PR adds a new class that transparently loads new pages as you iterate over a collection.

Highlights

Catching errors when decoding

json_decode() relies on manually checking error codes via json_last_error(). Guzzle used have a function to do this, but it was removed in Guzzle 6. 28929cf pulls in symfony/serializer so we can use it's JSON decoder. I know, it would be nice if it was it's own package, but I think the benefits of familiarity to Drupal developers outweighs the (small) cost of additional code in vendor.

A trait for rel link handling

44e6685 lets us share methods between the existing collection class and the new one.

A new interface for collections

e938b8c adds a new interface to the existing collection class. Technically there's a small BC break here because I added return typehinting to the \Iterator methods. I personally don't think it's worth rolling a new major release just for this given that code is likely broken if it breaks this contract anyways. If there's disagreement there, I'd be open to removing it or doing a little more refactoring that does break BC more directly.

Remaining Tasks

• Add a new client method to support using the paging collection 10b59d8
• Deprecate \Hussainweb\DrupalApi\Request\Request::getEntityClass() and move the responsibility to the request object. As is, it's brittle to extend this since it relies on specific class naming and FQCNs. I decided against this - I think being explicit about class mappings is more robust, but it isn't strictly required for this feature.