Add more clippy checks for better code/readability

A bit inspired by @paolobarbolini from this commit at lettre https://github.com/lettre/lettre/pull/784 .
I added a few more clippy lints here, and fixed the resulted issues.

Overall i think this could help in preventing future issues, and maybe
even peformance problems. It also makes some code a bit more clear.

We could always add more if we want to, i left a few out which i think
arn't that huge of an issue. Some like the `unused_async` are nice,
which resulted in a few `async` removals.

Some others are maybe a bit more estatic, like `string_to_string`, but i
think it looks better to use `clone` in those cases instead of `to_string` while they already are a string.

src/util.rs

@@ -300,7 +300,7 @@ impl Fairing for BetterLogging {
 //
 use std::{
     fs::{self, File},
-    io::{Read, Result as IOResult},
+    io::Result as IOResult,
     path::Path,
 };
 
@@ -309,11 +309,7 @@ pub fn file_exists(path: &str) -> bool {
 }
 
 pub fn read_file(path: &str) -> IOResult<Vec<u8>> {
-    let mut contents: Vec<u8> = Vec::new();
-
-    let mut file = File::open(Path::new(path))?;
-    file.read_to_end(&mut contents)?;
-
+    let contents = fs::read(Path::new(path))?;
     Ok(contents)
 }
 
@@ -326,11 +322,7 @@ pub fn write_file(path: &str, content: &[u8]) -> Result<(), crate::error::Error>
 }
 
 pub fn read_file_string(path: &str) -> IOResult<String> {
-    let mut contents = String::new();
-
-    let mut file = File::open(Path::new(path))?;
-    file.read_to_string(&mut contents)?;
-
+    let contents = fs::read_to_string(Path::new(path))?;
     Ok(contents)
 }