Add p5-Algorithm-Backoff, several classes that implement various backoff
strategies for setting delay between retry attempts:

- Algorithm::Backoff::Constant
- Algorithm::Backoff::Exponential
- Algorithm::Backoff::Fibonacci
- Algorithm::Backoff::LILD
- Algorithm::Backoff::LIMD
- Algorithm::Backoff::MILD
- Algorithm::Backoff::MIMD

(schmonz)