Fix clippy and rustfmt errors

[rileyshahar]

This is a chore PR to conform the codebase to clippy and rustfmt. I've made
it conformant in as frictionless a way as possible.

• Format code with rustfmt.
• Add basic CI.
• Fix enum variant capitalization.
• Make library re-exports public.
• Don't leak private types in APIs. Flags was previously private, but exposed
  in the public DataSource API.
• Allow module name repetitions for FileDataSource. It's possible we'll want
  to change the name of this object later, but this is the lowest-friction
  change.
• Clean up FileDataSource::new. It's more idiomatic to use provided methods on
  Option and Result than re-implenting the methods with match or if let.
• Make doc comments rustdoc-compatible. (Try running cargo doc --open!)
• Add must_use annotation to constructors. Any non-side-effecting function
  should always have this annotation.
• Use todo to panic at todo code sites. This is more idiomatic than writing a
  custom panic message.