[nit] Fix nits in fs.rs.

`std::fs::read_to_string` removes unnecessary boilerplate.

src/fs.rs

@@ -15,12 +15,10 @@ where
     P: AsRef<Path>,
 {
     let path = path.as_ref();
-    let mut f = File::open(path).map_err(|inner| Error::AccessError {
+    let string = std::fs::read_to_string(path).map_err(|inner| Error::AccessError {
         inner,
         path: path.to_owned(),
     })?;
-    let mut string = String::new();
-    let _ = f.read_to_string(&mut string);
     let result: A = serde_json::from_str(string.as_str()).map_err(|inner| Error::SerdeError {
         inner,
         path: path.to_owned(),
@@ -44,8 +42,7 @@ where
     fs::create_dir_all(path.as_ref()).map_err(|inner| Error::AccessError {
         inner,
         path: path.as_ref().to_owned(),
-    })?;
-    Ok(())
+    })
 }
 
 pub fn cp(from: &Path, to: &Path) -> Result<()> {