Sort top-level declarations topologically

It is quite annoying that all the helper functions have to always go before the place where they're used. Pretty much every language has some support for out-of-order definitions (e.g. Haskell does this automatically, C has forward declarations).

No files, tests, or entry points are named. Start by locating the compiler logic that processes top-level declarations and determine how dependencies between declarations are represented. Done means independent declarations can appear out of order while dependent declarations are processed according to their dependencies, with tests covering the behavior.

Written by the indexing model from the issue text.