Allow more lax usage

[sazzer]

I've just come across this and it's saved me writing it all out myself :)

However, there are two places where the types in this repository are more prescriptive than in the latest HAL spec:

• _links - the spec lists this as OPTIONAL, but the types in this repository have it as required.
• _links.self - the spec gives no mandate that a self link must be present, but the types in this repository are forcing it to be there.

The first of these means it's impossible to return a document that has _embedded or _templates without also including _links, and the second means it's impossible to return a document without a self link - e.g. the ephemeral response to a POST request might not have a self link but might still want to return other HAL-shaped details.

[evert]

Sounds reasonable!