Move `query()` funcs into separate structs

[wokket]

As part of moving to a 'toolbox' approach rather than all-in-one framework move the querying into it's own area (like EscapeSequence).

JsonPath uses a Selector struct with new() and find() funcs which I quite like.

[sempervictus]

Still trying to make sense of what benefit this brings, other than code clarity and a refactor opportunity.

[wokket]

I haven't done any serious work here yet, but that pretty much sums it up... hoping for :

• Reduce amount of code in Message etc for clarity/easier modification
• Separation of Concerns
• Keep query tests all together
• Better fit 'toolkit' approach
• More opportunity to optimise perf by caching data in the selector (like the regexes in EscapeSequence)