Destructible v7.0.0-alpha.37

• Test awaiting done.
• Tidy sub-destructible option parsing.
• Test set sub-destructible timeout.
• Test rescue of a promise.
• Destroy deferrables if errored.
• Ephemeral timer takeover.
• On error return for sub-destructibles.
• Use Future in shutdown.
• New constructor arguments.
• Remove trace function check.
• Remove exceptional.
• Auto-destroy rescue, remove terminal.

Closes #253.