Reorganize some of the unwieldly parts of the codebase

* Moved commands into their own folder to reduce `bin.rs`'s size
* Moved all of the VFS-related functionality into its own folder
* Documented a lot of functions, including a few very obscure ones

src/vfs/vfs_item.rs

@@ -0,0 +1,31 @@
+use std::collections::HashMap;
+
+/// A VfsItem represents either a file or directory as it came from the filesystem.
+///
+/// The interface here is intentionally simplified to make it easier to traverse
+/// files that have been read into memory.
+#[derive(Debug, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase", tag = "type")]
+pub enum VfsItem {
+    File {
+        route: Vec<String>,
+        contents: String,
+    },
+    Dir {
+        route: Vec<String>,
+        children: HashMap<String, VfsItem>,
+    },
+}
+
+impl VfsItem {
+    pub fn name(&self) -> &String {
+        self.route().last().unwrap()
+    }
+
+    pub fn route(&self) -> &[String] {
+        match self {
+            &VfsItem::File { ref route, .. } => route,
+            &VfsItem::Dir { ref route, .. } => route,
+        }
+    }
+}