Amalgamate v0.1.55

• Simplified shutdown, drain() becomes dubious. Seems like, if we get rid of isDestroyedIfDestroyed we can shutdown by
• Upgrade skip to 0.2.36.
• Upgrade b-tree to 2.0.0-alpha.97.
• Upgrade splice to 3.0.0-alpha.55.
• Upgrade fracture to 0.3.0-alpha.50.
• Upgrade destructible to 7.0.0-alpha.49.
• Upgrade turnstile to 6.0.0-alpha.58.
• Upgrade riffle to 2.0.0-alpha.71.

Closes #224.