Develop minimal plan for exposing MatchingContext over FFI.

[alilleybrinker]

This is part of #67

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MatchingContext is a type used for all the individual matching functions, and we want users to be able to call those functions over FFI, so we need to have some plan to expose it. Unfortunately, it's big and complicated, so I am inclined to come up with a minimal set of things we can expose to make the relevant functions callable, and then come back and enhance how much we're exposing in the future.

This issue is tracking the development of that plan. Once we have a plan, we can consider this resolved and open a new issue.

[alilleybrinker]

Looking at things in more detail:

The first question is how to permit construction of MatchingContext. Thankfully, we've already exposed PactSpecification over FFI, and DiffConfig is a simple enum we can also expose. So we can start by exposing the two most basic constructors: MatchingContext::default and MatchingContext::with_config. However, neither of these has any matching rules associated with them, so they're not useful.

So then we'll need to expose MatchingRuleCategory. Similar, this has some basics that can be done first. MatchingRuleCategory::empty and MatchingRuleCategory::equality are both easy to do up front. MatchingRuleCategory::from_json and MatchingRuleCategory::add_rules_from_json are a possibility, and requires RuleLogic, which is just a simple enum, easy to expose. With those, and some other accessor methods and things, I think we can have our basics covered.

So the very first pass gives us:

• Expose RoleLogic
• Expose DiffConfig
• Expose MatchingContext::default
• Expose MatchingContext::with_config
• Expose MatchingRuleCategory::empty
• Expose MatchingRuleCategory::equality
• Expose MatchingRuleCategory::rule_from_json
• Expose MatchingRuleCategory::add_rules_from_json

[alilleybrinker]

Started implementation of this, but for some reason cbindgen isn't running. Trying to upgrade cbindgen to the latest version is stalling out with network issues, which after an hour I haven't resolved. I'll come back to it on Monday.

[alilleybrinker]

Still haven't resolved my cbindgen issues, but I'm forging ahead with the code regardless.

[alilleybrinker]

This work is happening in the basic_matching_context branch.