Amalgamate v0.1.60

• Upgrade skip to 0.2.37.
• Upgrade b-tree to 2.0.0-alpha.98.
• Upgrade homogenize to 2.0.0-alpha.19.
• Upgrade writeahead to 0.0.28.
• Upgrade splice to 3.0.0-alpha.56.
• Upgrade fracture to 0.3.0-alpha.51.
• Upgrade destructible to 7.0.0-alpha.50.
• Upgrade turnstile to 6.0.0-alpha.60.
• Upgrade riffle to 2.0.0-alpha.72.
• Wait for merge to flush before committing.

Closes #229.