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.md

@@ -27,6 +27,6 @@ fn main() {
 
 * Packages provide functionality and include a `Cargo.toml` file that describes how to build a bundle of 1+ crates.
 * Crates are a tree of modules, where a binary crate creates an executable and a library crate compiles to a library.
-* Modules define organization, scope, and is the focus of this section.
+* Modules define organization, scope, and are the focus of this section.
 
 </details>

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>

src/std/hashmap.md

@@ -47,5 +47,20 @@ fn main() {
       .or_insert(374);
   ```
 * Unlike `vec!`, there is unfortunately no standard `hashmap!` macro.
+  * Although, since Rust 1.56, HashMap implements [`From<[(K, V); N]>`][1], which allows us to easily initialize a hash map from a literal array:
+
+  ```rust,ignore
+  let page_counts = HashMap::from([
+    ("Harry Potter and the Sorcerer's Stone".to_string(), 336),
+    ("The Hunger Games".to_string(), 374),
+  ]);
+  ```
+
+ * Alternatively HashMap can be built from any `Iterator` which yields key-value tuples.
+* We are showing `HashMap<String, i32>`, and avoid using `&str` as key to make examples easier. Using references in collections can, of course, be done,
+  but it can lead into complications with the borrow checker.
+  * Try removing `to_string()` from the example above and see if it still compiles. Where do you think we might run into issues?
+
+[1]: https://doc.rust-lang.org/std/collections/hash_map/struct.HashMap.html#impl-From%3C%5B(K%2C%20V)%3B%20N%5D%3E-for-HashMap%3CK%2C%20V%2C%20RandomState%3E
 
 </details>

src/std/string.md

@@ -27,8 +27,7 @@ fn main() {
 
 <details>
 
-* `len` returns the size of the `String` in bytes, not its length in characters.
-* `chars` returns an iterator over the actual characters.
-* `String` implements `Deref<Target = str>` which transparently gives it access to `str`'s methods.
+* `len()` returns the size of the `String` in bytes, not its length in characters.
+* `chars()` returns an iterator over the actual characters.
 
 </details>