Release 1.5.0

• Fix Backoff docs (and jitter logic).
• Add short-circuit to Sleep(d) to return immediately if d is 0.
• go.mod: Update Go version: 1.12 -> 1.20
• Add functions CalculateBackoff and MustCalculateBackoff
• Add Backoff options for StartWait, InitialWait, and WithTotalDelay