Introduce NodeIdentifiers

[brotkrueml]

It is possible in JSON-LD to use only a @id reference for a property. This is useful when using circular references, e.g.:

{
    "@context": "http://schema.org/",
    "@type": "Person",
    "@id": "https://example.org/#agent1",
    "name": "Secret Agent 1",
    "knows": {
        "@type": "Person",
        "name": "Secret Agent 2",
        "knows": { "@id": "https://example.org/#agent1" }
     }
}

Or when using types which are connected with isPartOf and hasPart properties.

Also JSON-LD knows the concept of a blank node identifiers. This is useful when the id has no IRI attached to, but nodes should be connected:

{
    "@context": "http://schema.org/",
    "@type": "Person",
    "@id": "_:b1",
    "name": "Secret Agent 1",
    "knows": {
        "@type": "Person",
        "name": "Secret Agent 2",
        "knows": { "@id": "_:b1" }
     }
}

The blank node identifier starts with _:.

• [ ] A NodeIdentifier class and a BlankNodeIdentifier class are available.
• [ ] An identifier can be given on instantiation of the NodeIdentifier, it is immutable.
• [ ] The identifier is generated automatically on instantiation of the BlankNodeIdentifier, it is immutable.
• [ ] Both classes have a method to get the id.
• [ ] A NodeIdentifier/BlankNodeIdentifier can be assigned to a property as a value.
• [ ] Instead of giving a string to the setId() method of a type also a NodeIdentifier/BlankNodeIdentifier can be used.
• [ ] On rendering the @id value is set as defined in a type/property.
• [ ] A NodeIdentifier/BlankNodeIdentifier can also be used as ViewHelpers.
• [ ] The feature is described in the documentation.
• [ ] The changed method signatures are described in the documentation.
• [ ] An entry to the changelog is added.