Rewrite to use walkdir instead of recursion

Advised to use walkdir by burntsushi as using recursion on file systems
can blow the stack.

walkdir is slower but allows the code to be cleaner and more reliable

Also experimented with ignore but locking the hashmap resulted in
similar performance to walkdir but with much uglier code.

src/main.rs

@@ -1,15 +1,14 @@
 #[macro_use]
 extern crate clap;
 extern crate assert_cli;
+extern crate walkdir;
 
 use self::display::draw_it;
 use clap::{App, AppSettings, Arg};
 use utils::{find_big_ones, get_dir_tree};
 
-
 mod display;
 mod utils;
-mod lib;
 
 static DEFAULT_NUMBER_OF_LINES: &'static str = "15";
 
@@ -40,9 +39,9 @@ fn main() {
     let number_of_lines = value_t!(options.value_of("number_of_lines"), usize).unwrap();
     let use_apparent_size = options.is_present("use_apparent_size");
 
-    let (permissions, node_per_top_level_dir) = get_dir_tree(&filenames, use_apparent_size);
-    let slice_it = find_big_ones(&node_per_top_level_dir, number_of_lines);
-    draw_it(permissions, &node_per_top_level_dir, &slice_it);
+    let (permissions, nodes) = get_dir_tree(&filenames, use_apparent_size);
+    let biggest_ones = find_big_ones(nodes, number_of_lines);
+    draw_it(permissions, filenames, biggest_ones);
 }
 
 #[cfg(test)]