use the JSON primitive types for Boolean, Integer, Number in compaction output

[fennibay]

Currently JSON-LD compaction always outputs the values as strings, even when the @type is specified to be a primitive type such as xsd:boolean, xsd:integer and xsd:double.

It would be nicer for JSON parsers that such values would be available as native JSON Booleans, Integers and Numbers.

Basically I'm proposing to introduce a useNative flag in compaction options, similar to fromRdf and Normalize.

See also:

• Add support for xsd:float as native type #462
• Should the JSON-LD API have a "mode": "strict" flag? json-ld/json-ld.org#100
• Use of native types in from/to RDF json-ld/json-ld.org#150

edit: removed Null from the proposal, that doesn't have a corresponding xsd type.