Use CoreML for on-device recommendations

[tanmayb123]

In Core ML 2 (coming in September), Apple has added support for custom models. This means that recommender systems trained in Turi Create v5 are compatible with Core ML 2 / Xcode 10 / macOS Mojave / iOS 12 / watchOS 5 / tvOS 12.

Because these versions are currently in beta, I'll create a branch with code that works with Core ML 2, but will merge with master in September, when everything's stable.

There are also a few issues that the Turi Create team is working around in terms of custom models and getting recommender systems to work with Core ML 2.