Fix massive memory allocation issue

Not exactly sure what was going on, other than cloning of the FileMatch
with its results, since it is recursive I believe this blew up the
stack/heap with unnecessary allocations. By using an iterator and filter
this seemed to greatly reduce memory use and improve speed.

src/main.rs

@@ -19,26 +19,32 @@ fn main() {
     }
 }
 
-fn traverse_dir(config: &config::Config, dir: &Path, operation: fn(&config::Config, &Path) -> io::Result<FileMatch>)
-    -> io::Result<Vec<FileMatch>> {
-
+fn traverse_dir(
+    config: &config::Config,
+    dir: &Path,
+    operation: fn(&config::Config, &Path) -> io::Result<FileMatch>,
+) -> io::Result<Vec<FileMatch>> {
     let mut matches: Vec<FileMatch> = Vec::new();
     for entry in fs::read_dir(dir)? {
         let entry = entry?;
         let file_type = entry.file_type()?;
 
         if file_type.is_dir() {
-                let results = traverse_dir(&config, &entry.path(), operation)?;
-                for mat in results.as_slice() {
-                    if mat.matches.len() > 0 {
-                        matches.append(&mut results.clone());
-                    }
+            let mut results = traverse_dir(&config, &entry.path(), operation)?;
+            let has_results = results
+                .iter()
+                .filter({ |mat| mat.matches.len() > 0 })
+                .count()
+                > 0;
+
+            if has_results {
+                matches.append(&mut results);
             }
         } else {
             let result = operation(&config, &entry.path());
             match result {
                 Result::Ok(result) if result.matches.len() > 0 => matches.push(result),
-                    _ => ()
+                _ => (),
             }
         }
     }
@@ -46,7 +52,7 @@ fn traverse_dir(config: &config::Config, dir: &Path, operation: fn(&config::Conf
     Ok(matches)
 }
 
-#[derive(Debug, Clone)]
+#[derive(Debug)]
 struct FileMatch {
     file_name: String,
     matches: Vec<Match>