wip: Support for fn eval()

[deepakraog]

Add support for JSONata string functions #92

• $eval()

[tommy]

I think we need a different approach to implementing $eval. The current direction appears overfitted to the test cases, rather than a general solution. Any jsonata expression can be used inside the string argument to $eval (including eval itself), so I think we need to somehow invoke the complete parser/evaluator.

I don't know off the top of my head what level is the right level to hook into. It could be creating an entirely new crate::JsonAta instance. We'll need to find the right way to pass in the bindings, too, since by default the string is evaluated in the current expression's context.