B-tree v2.0.0-alpha.23

• Upgrade ascension to ^1.2.0. Closes #618.
• Do not perform final binary search on leaf. Closes #617.
• Rmeove unnecessary wrapping from unit tests.
• Remove unlocker. Closes #616. Closes #612.
• Require that the user extract the key. Closes #615.
• Expose comparator and extractor. Closes #614.

Closes #619.