perf: remove toSourceRel

toSourceRel is an expensive operation since it iterates over all the
relationships detected. We now use an alias on the CTE so embedding can
work without toSourceRel.

src/PostgREST/Query/Statements.hs

@@ -6,8 +6,6 @@ This module constructs single SQL statements that can be parametrized and prepar
 
 - It consumes the SqlQuery types generated by the QueryBuilder module.
 - It generates the body format and some headers of the final HTTP response.
-
-TODO: Currently, createReadStatement is not using prepared statements. See https://github.com/PostgREST/postgrest/issues/718.
 -}
 module PostgREST.Query.Statements
   ( createWriteStatement