Amalgamate v0.1.38

• Upgrade designate to 2.0.0-alpha.13. Closes #206.
• Upgrade splice to 3.0.0-alpha.31. Closes #205.
• Upgrade homogenize to 2.0.0-alpha.13. Closes #204.
• Upgrade riffle to 2.0.0-alpha.45. Closes #203.
• Upgrade b-tree to 2.0.0-alpha.66. Closes #202.
• Upgrade skip to 0.2.11. Closes #201.
• Upgrade ascension to 3.0.0-alpha.0. Closes #200.
• Fix get candidate selection. Closes #199.
• More thoughts on the compound key requirement.

Closes #207.