Spec compliance bug: Links object member names must not use colons

Problem/Motivation

The allowed member names for JSON:API objects (i.e. JSON object property names) has an allowed character set. This includes letters, numbers, dashes, underscores, and spaces. Notably, colons are explicitly forbidden.

Unfortunately, JSON:API can serialize multiple links having the same key using a colon. This can happen if multiple resources are omitted from the response for access reasons.

Proposed resolution

Change the serialization to use a double dash (--) instead of a colon (:).

User interface changes

None.

API changes

None. See jsonapi.api.php:

 * HTTP API: URLs and JSON response structures are considered part of this
 * module's public API. However, inconsistencies with the JSON:API specification
 * will be considered bugs. Fixes which bring the module into compliance with
 * the specification are *not* guaranteed to be backwards-compatible. When
 * compliance bugs are found, clients are expected to be made compatible with
 * both the pre-fix and post-fix representations.

Data model changes

None.

Release notes snippet

JSON:API omission links are now keyed using a double-dash-separated syntax instead of a colon-separated syntax in order to be brought into compliance with the JSON:API specification on allowed member names. This may require changes to client applications that are parsing document omission links. See the change record on the change to object omission link serialization for an example.