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

src/node.rs

@@ -0,0 +1,54 @@
+use crate::platform::get_metadata;
+
+use std::cmp::Ordering;
+use std::path::PathBuf;
+
+#[derive(Debug, Eq, Clone)]
+pub struct Node {
+    pub name: PathBuf,
+    pub size: u64,
+    pub children: Vec<Node>,
+    pub inode_device: Option<(u64, u64)>,
+}
+
+pub fn build_node(
+    dir: PathBuf,
+    children: Vec<Node>,
+    use_apparent_size: bool,
+    by_filecount: bool,
+) -> Option<Node> {
+    match get_metadata(&dir, use_apparent_size) {
+        Some(data) => {
+            let (size, inode_device) = if by_filecount { (1, data.1) } else { data };
+            Some(Node {
+                name: dir,
+                size,
+                children,
+                inode_device,
+            })
+        }
+        None => None,
+    }
+}
+
+impl PartialEq for Node {
+    fn eq(&self, other: &Self) -> bool {
+        self.name == other.name && self.size == other.size && self.children == other.children
+    }
+}
+
+impl Ord for Node {
+    fn cmp(&self, other: &Self) -> Ordering {
+        if self.size == other.size {
+            self.name.cmp(&other.name)
+        } else {
+            self.size.cmp(&other.size)
+        }
+    }
+}
+
+impl PartialOrd for Node {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        Some(self.cmp(other))
+    }
+}