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
