Move base_uri from Parser/Serializer config to document prop

[damooo]

Current parsers and serializers are configured with base_uri as their property.
And thus we have to instantiate those for every doc with different uri.

Instead it would be natural to take base_uri as an attribute of document to be parsed/serialized. Thus one parser/serializer instance can operate over any number of docs. Essentially uri is a property of the document.

For example, json-ld interface defines a RemoteDocument abstraction, as a product of uri, content, content-type of a document. This seems a good abstraction.

[pchampin]

BufRead is very pervasive in Rust, so I would rather keep them as a possible input for parsers. And therefore (since BufReads don't carry their origin URL), having a default base IRI in the parser configuration makes sense.

But that being said, having an abstraction similar to JSON-LD's RemoteDocument, that attaches metadata (such as origin URL) to the data to be parsed, sounds indeed like a good idea. Parsers implementations could accept either BufRead of this abstraction.