VariablesSkipAndFragmentElaborator

A phase which elaborates variables, directives, fragment spreads and inline fragments.

1. Query variable values are substituted for all variable references.

2. skip and include directives are handled during this phase and the guarded subqueries are retained or removed as appropriate.

3. Fragment spread and inline fragments are expanded.

4. types narrowing coercions by resolving the target type against the schema.

5. verifies that leaves have an empty subselection set and that structured types have a non-empty subselection set.