feat: Evaluate JSON Pointers in Policy Expressions

[cstepanian]

Relates to #371

This PR replaces the previous preprocessor-based implementation of JSON Pointers with one that natively runs in the Policy Expression Executor.

It uses the ExprMutator visitor trait to evaluate JSON Pointers in two steps: first, pointers are looked up but the original pointer string is preserved. Then, the looked-up value is inserted. This structure is meant to allow for better error messages and user-facing explanations of why a Policy Expression was evaluated a given way.

[j-lanson]

Hey Cal,

Your solution right now is very reasonable, and I don't mind merging it as-is and making changes to it in a subsequent PR.

As for your questions about passing around the context, let me know if this is a possible solution.

The way I was imagining it, you have an Expr that represents the parsed program, which may contain JsonPointer variants. You have a function preprocess(expr: &mut Expr, context: &Value) which produces an Expr tree guaranteed to have no JsonPointer variants. You can then execute the program as normal without having to pass around &context.

It's like the JSON pointers are macros, but they get replaced once we have a program AST instead of before everything is tokenized.

[alilleybrinker]

Discussed, plan is to merge #458 and then rebase this on top and get it merged.

[cstepanian]

Re-implemented with late binding and using the ExprMutator visitor trait.

@j-lanson @alilleybrinker Requesting review again.

[j-lanson]

Nice, looks very clean