Cleans up Next, Previous and Closest by integrating SearchingAdjuster and subclasses
Adds CurrentOrPrevious anchor
Adds a few boundary tests for Previous, CurrentOrPrevious and Next
Adds CurrentField for expressions like 'this week'. Could be used to replace Today, but Today is such a common concept that it's probably worth keeping it as its own thing.
Adds support for PAST_REF and FUTURE_REF
Generalizes Date to allow just a year, a year and a month, etc.