Support relation keys with colons or other special symbols in them

In HAL, curies are used as a mechanism to namespace relations. The namespace is prepended with a colon in front of the relation name, as can be seen in this example in traverson-hal: https://www.npmjs.com/package/traverson-hal#usage (they visit the relations `ht:me` and `ht:posts`).

Since we use relation names as identifiers, we currently do not support this kind of relation names. This issue is meant as a space to discuss the best way to implement this kind of relation namespacing.

Side note: I haven't checked, but `hal-json-normalizer` should already support this as far as I am aware, since it is only concerned with string keys in JSON.