Survey implementations of context capabilities in other languages

[yoshuawuyts]

We should create a section summarizing how the problem we're setting out to solve here is solved in other languages. For example Koka's effects, Scala's implicits (versions 1 and 2), and likely more. We should be able to articulate how our design differs from similar designs in other languages, and why our design works best for Rust.

[yoshuawuyts]

• Hack: Contexts and Capabilities
• Koka: effects
• Coq: implicit arguments
• Agda: implict arguments
• Scala: implicit parameters
• Dagger: dependency injection