Large refactor. Use rayon, 10X performance boost

Code changes:
Removed ignore & channel crates. Using a single reciever thread to build
a hashmap to prevend duplicate inodes being reported gave a severe
performance penalty

Using rayon crate with some hand crafted file traversal has improved
performance aprox 10X

Behaviour changes:

Removed parameter 'limit by filesystem' - don't think this is used, and
I only added it as it was easy to add with the ignore crate.

Sym links will now not appear in the output tree unless using '-s'
'apparent-size' flag

Change behaviour of multiple args so that it unifies them and
compares them under one tree instead of treating them
individually: https://github.com/bootandy/dust/issues/136

tests/test_flags.rs

@@ -142,8 +142,8 @@ pub fn test_number_of_files() {
         .unwrap()
         .stdout;
     let output = str::from_utf8(&output).unwrap();
-    assert!(output.contains("1     ┌── hello_file"));
-    assert!(output.contains("1     ├── a_file "));
+    assert!(output.contains("1     ┌── a_file "));
+    assert!(output.contains("1     ├── hello_file"));
     assert!(output.contains("3   ┌─┴ many"));
     assert!(output.contains("4 ┌─┴ test_dir"));
 }