Improvements to serialisation

[Jym77]

JSON:

• "short" option for JSON serialisation that turns Element into their path, rather than their sub-tree.

EARL:

• Make sure that each EARL output is as complete as possible, the graph should be compacted afterwards.
• Have an Assertor EARL pre-baked, to be used in reports.

New:

• Add a "JS path" serialisation for DOM tree, that can access any node (including into shadow DOM or content document). This would be a JS eval(…)-able expression to select it, such as document.childNodes[2].shadowRoot. … In contexts where eval cannot be used (e.g. content scripts injected from an extension), these are easy to parse and follow (and we can provide the parser too).