Top-level symbols are mishandled during compilation

[Achille]

The current code only retains function declarations, and may duplicate symbols that exist in files that don't have a //go:build !durable tag.

We need to be able to retain all top-level declarations in the compiled file (e.g. const, var, etc...), and have a strategy for preventing symbol duplication; for example, we could require //go:build !durable on all files, or ignore symbols in files that don't have the build tag.