Caching inner schemas

[pd]

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I've been working on using json-schema to validate all incoming JSON data for an API I'm writing; it's functional, but can be pretty slow for complex endpoints (50ms+). I did some profiling to narrow it down to hitting JSON::Schema#initialize a lot. The change in #180 helped a bit, but the core problem remains that we constantly re-create Schema instances.

I plan to add either Schema#inner_schemas['#/some/path/fragment'], or even just directly register them into JSON::Validator.schemas; the various Attribute validation classes can then fetch from that, and only create a new schema on miss.

The code in this PR adds rspec for now, only because I couldn't come up with any more reasonable means of testing this right now. I've not touched lib at all yet.