clippy cleanup #2

- remove needless return
- remove needless format!
- remove a few more needless clone()
- from_str_radix(_, 10) -> .parse()
- remove needless reference
- remove needless `mut`

Also manually replaced a match statement with map_err() because after
clippy was done with it, there was almost nothing left in the match
expression.

zenith/src/main.rs

@@ -76,7 +76,7 @@ fn main() -> Result<()> {
 
     // all other commands would need config
 
-    let repopath = PathBuf::from(zenith_repo_dir());
+    let repopath = zenith_repo_dir();
     if !repopath.exists() {
         bail!(
             "Zenith repository does not exists in {}.\n\