[Feature] Use the newer file path style

[acoglio]

According to this text in the Rust Book, the currently preferred path for the code in a module mymodule is .../mymodule.rs rather than .../mymodule/mod.rs. As noted in the referenced text, this avoids the proliferation of many files called mod.rs, which makes the files harder to spot and distinguish quickly in an IDE with many open editors (and I've in fact experienced that).