Script-aware fallback

Clearly one of the trickier aspects to integrating with a text layout system is script aware fallback. Issue #28 is one way, provide metadata. The problem is, it's not really supported by platform text libraries.

What is supported, and used by most web browsers, is an API for choosing a fallback font based on a Unicode code point and other data, generally locale preferences. In CoreText, that's the not-well-documented CTFontCopyDefaultCascadeListForLanguages. In DirectWrite, it's IDWriteFontFallback with IDWriteTextAnalysisSource. In discussions with @pcwalton and further thought, I think the best thing to do is to implement an abstraction for those API's in font-kit.

This will change and increase the API surface of font-kit quite a bit. In particular, we need to adopt a locale representation, and have code to plumb that to platform API calls. I suggest we use fluent-locale-rs, as this has promise to be a standard across the Rust ecosystem.

Discussion is welcome, as is an offer to implement the functionality. However, I'm also quite willing to do it myself, especially as I expect this to interact with the design of the rest of skribo, including its API.

Start by reviewing the proposed abstraction around CoreText's CTFontCopyDefaultCascadeListForLanguages and DirectWrite's IDWriteFontFallback with IDWriteTextAnalysisSource. Examine how fluent-locale-rs could represent locale preferences and how those values would flow into platform API calls. Done means a cross-platform font fallback API is designed and implemented, with the resulting API surface fitting the rest of font-kit and skribo.