In SQLite, the field equivalent to `constraint_usage` (`aConstraintUsage`
from `sqlite3_index_info`) is used to request arguments that are later
passed to the `xFilter` method. In Limbo, this behavior applies to
virtual tables, but not to table-valued functions. Currently, TVFs have
dedicated handling that passes all function arguments to the filter
method and doesn't use information provided in the `constraint_usage`
field.
This commit is a step toward unifying the handling of virtual tables and
TVFs.
631d62e9f8