Day2: Afternoon - speaker note details and minor cosmetic changes. (#408)

* A few speaker notes in Day2: Afternoon and minor cosmetic changes.

* Do not test filesystem example code block.

src/modules/filesystem.md

@@ -20,3 +20,34 @@ The `crate` root is in:
 
 * `src/lib.rs` (for a library crate)
 * `src/main.rs` (for a binary crate)
+
+<details>
+
+* The change from `module/mod.rs` to `module.rs` doesn't preclude the use of submodules in Rust 2018.
+  (It was mandatory in Rust 2015.)
+
+  The following is valid:
+
+  ```ignore
+  src/
+  ├── main.rs
+  ├── top_module.rs
+  └── top_module/
+      └── sub_module.rs
+  ```
+
+* The main reason for the change is to prevent many files named `mod.rs`, which can be hard
+  to distinguish in IDEs.
+
+* Rust will look for modules in `modulename/mod.rs` and `modulename.rs`, but this can be changed
+  with a compiler directive:
+
+  ```rust,ignore
+  #[path = "some/path.rs"]
+  mod some_module { }
+  ```
+
+  This is useful, for example, if you would like to place tests for a module in a file named
+  `some_module_test.rs`, similar to the convention in Go.
+
+</details>