Strata v2.0.0-alpha.119

• Remove leaf/branch comparator distinction. Closes #788.
• User must provide extractor and comparator. See #788.
• Parital search. See #788.
• Partition using a partition length.
• Composite keys only. See #788.
• Fix comparison against null.
• Upgrade reciprocate to 0.0.9.
• Preparing for a change to array keys only.

Closes #789.