Separate tracking structs from compile-time dependencies

[michalmuskala]

This is a first step to limiting recompilations in case a struct is used in a
module, the "parent module" of the struct changes, but struct definition itself
does not change.

Things left to do outside this PR:

• provide module.__info__(:struct_md5)
• take advantage of the new information in Mix.Compilers.Elixir
• update mix xref task.

[ericentin]

Is module.__info__(:struct_md5) intended to be the md5 of the struct spec to be used to determine whether a struct's definition has changed? Perhaps phash2 would be acceptable for this use case and also perhaps more OTP-native?

[josevalim]

@antipax phash2 main focus is a proper hash distribution, so it is useful on things such as HashDict. I think md5 will do a better job of avoiding collisions.

[josevalim]

I have merged this manually but there is still a bit more of work to be done. We need to add struct_md5 and we need to consider it in the compiler. Thanks for the iniciative and the idea on this! ❤️