library: migrate public/feature layer of iOS library to Swift

[goaway]

We've decided to implement our public and platform-specific library layer primarily in Swift on iOS. Migrating to this has some repercussions for the build and for the time-being will require custom Bazel rules to compose the distributable static framework.

[goaway]

As an update, currently working to get a swift-based framework target functional in Bazel. Have a target that builds (working off of a similar rule created by @keith), but it looks like dependencies aren't getting bundled. Still, making progress and continuing to update #188.

[keith]

Our rule by design doesn't flatten the deps. So we'll have to figure that out.

[junr03]

Done by #188