Address type-level crashes

[tkellogg]

This project is written in Rust so that the daemon can be very reliable. However, we have to actually take advantage of Rust's features to do that.

1. Remove all .unwrap() calls
2. Make cargo clippy pass

[tkellogg]

I made a little bit of progress in d32a7cf. The logging code in poller.rs now does a decent job of avoiding .unwrap().

[tkellogg]

Current problems:

➜  dura git:(master) rg 'unwrap|expect'
src/main.rs
11:    let dir = std::env::current_dir().unwrap();
14:            if let Some(oid) = snapshots::capture(&dir).unwrap() {
38:    config.set_watch(path.to_str().unwrap().to_string(), WatchConfig::new());

src/log.rs
68:        let path = Config::default_path().parent().unwrap().join(file_name);
78:                eprintln!("Unable to write file {}", path.to_str().unwrap_or("n/a"));

src/snapshots.rs
41:        Some(&branch_commit.as_ref().unwrap_or(&head).tree()?),
58:        &[ branch_commit.as_ref().unwrap_or(&head) ],

src/poller.rs
23:    let repo = path.to_str().unwrap_or("<invalid path>").to_string();

src/config.rs
35:        .expect("Could not find your home directory!")
41:        Self::load_file(Self::default_path().as_path()).unwrap_or(Self::empty())
58:            .unwrap();
61:        serde_json::to_writer(writer, self).unwrap();
62:        println!("Wrote {}", path.to_str().unwrap());