Fix all lints that are new with Rust 1.54

They are all of the following type:
https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrow

src/bin/bat/app.rs

@@ -301,7 +301,7 @@ impl App {
                     .map(|style_str| {
                         style_str
                             .split(',')
-                            .map(|x| StyleComponent::from_str(&x))
+                            .map(|x| StyleComponent::from_str(x))
                             .collect::<Result<Vec<StyleComponent>>>()
                     })
                     .transpose()?;

src/bin/bat/assets.rs

@@ -25,7 +25,7 @@ pub fn clear_assets() {
 
 pub fn assets_from_cache_or_binary(use_custom_assets: bool) -> Result<HighlightingAssets> {
     let cache_dir = PROJECT_DIRS.cache_dir();
-    if let Some(metadata) = AssetsMetadata::load_from_folder(&cache_dir)? {
+    if let Some(metadata) = AssetsMetadata::load_from_folder(cache_dir)? {
         if !metadata.is_compatible_with(crate_version!()) {
             return Err(format!(
                 "The binary caches for the user-customized syntaxes and themes \
@@ -42,7 +42,7 @@ pub fn assets_from_cache_or_binary(use_custom_assets: bool) -> Result<Highlighti
     }
 
     let custom_assets = if use_custom_assets {
-        HighlightingAssets::from_cache(&cache_dir).ok()
+        HighlightingAssets::from_cache(cache_dir).ok()
     } else {
         None
     };

src/bin/bat/main.rs

@@ -226,7 +226,7 @@ pub fn list_themes(cfg: &Config) -> Result<()> {
 
 fn run_controller(inputs: Vec<Input>, config: &Config) -> Result<bool> {
     let assets = assets_from_cache_or_binary(config.use_custom_assets)?;
-    let controller = Controller::new(&config, &assets);
+    let controller = Controller::new(config, &assets);
     controller.run(inputs)
 }