"collection" and "item" link relation types

[handrews]

These link relation types from RFC 6573 implement one of the most common patterns in REST APIs. Appropriately, the RFC does not discuss HTTP-specific behavior for these links, but provides two media types as reference uses which both specify the create-on-POST pattern for the relations.

We should add similar language for using these relations in the context of application/schema+json Hyper-Schemas.

There is some dependency here on #140 (Link source / "anchor" / "anchorPointer") for properly setting the context of an "item" link to the entire collection and not just the array element or other nested structure representing the idem in the collection.

[handrews]

I'm going to write up a PR for this as soon as #140 is resolved enough to make this workable. Right now there's no way to describe the proper behavior (which is a problem even without these link relations, but it makes these impossible to write up in detail).

[handrews]

Merged PR #412.