Add a new operator to make an HKeyRow from expressions

Such a row is suitable for use by GroupLookup, meaning that this operator allows a query to get data when the primary key is known exactly, without going through the PRIMARY index. More precisely, it is the entire HKey that must be known, which can mean the primary key, the primary key and parent foreign key (or its column equivalence from an INNER JOIN), or the primary key, parent foreign key and grandparent primary key that does not even appear in the grandchild row.

See new tests for examples.