Adding back-off instead of busy-spinning

Also changed the total number of spinning similar to Cilk
Could you add a comment saying where this came from? Was this from Cilk, or from performance testing, etc…?

Also, no need for parentheses around `if` conditions in Rust, just write this as `if i > 100`.

Additionally, I would make 100 into a constant and hoist it to the top of the file.