R1:0511b2a0129e

Implement the "present" and "absent" operators in the Ferret execution engine

Summary:
Ref T13509. Now that the compiler can parse these queries, actually implement them.

These are fairly easy to implement:

- For present, just "JOIN". If it works, the field is present.
- For absent, we "LEFT JOIN" and then "WHERE any_column IS NULL".

Test Plan: Searched for various documents with and without fields present, got sensible results in Maniphest. For example, "body:-" finds tasks with no body, "body…