Support Skolemization

[gkellogg]

Add Enumerable#skolemize, which takes an argument for the .well-known/genid baseURI, and a block or returns an Enumerator. The block yields each statement with BNodes replaced with URIs to those BNodes.

Depends on a :skolemize feature supported by the Enumerable.

URIs are extended with information to identify the specific BNode associated with the URI.

If the Enumerable is also Mutable, then the Enumerator returned from #skolemize is also mutable, and updates involving skolemized URIs are made against the base Enumerable after substituting involved skoleum URIs with their associated BNodes.

@no-reply does this seem like what you need for LDP? @bendiken thoughts?

May want a sub-class of URI to add skolum behavior.

[gkellogg]

See ruby-rdf/rdf-ldp#35

[gkellogg]

• gkellogg: the only other reason to push this is conceptual purity
  [2:50pm] tjohnson: * bendiken: it could be quite painful externally