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/mod.rs

@@ -0,0 +1,7 @@
+mod vfs_session;
+mod vfs_item;
+mod vfs_watcher;
+
+pub use self::vfs_session::*;
+pub use self::vfs_item::*;
+pub use self::vfs_watcher::*;