Write a system for emitting warnings from the compiler

A predecessor to #338.

Pretty much every well-known compiler can emit many warnings that hint at potential user errors. We don't have any of those yet, but we should build an extensible infrastructure that enables us to emit them.

Start by reviewing predecessor issue #338 and the compiler-related parts of the repository to understand the existing diagnostic flow. Determine the scope of an extensible warning-emission infrastructure and what compiler behavior would demonstrate that it is complete.

Written by the indexing model from the issue text.