schemas/util.sql: added runnable_sql()

fix: schemas/util.sql: eval2*(): added search_path function option in order to limit the effects of any `SET LOCAL search_path` in the invoked query to the current function. however, plain eval() is not changed because it is often used to execute a `SET LOCAL search_path` in the calling function.

schemas/util.sql: diff(text, text): removed unnecessary casts to the base type in the join condition and the WHERE filter. these had been presumed necessary due to errors, but the errors turned out to be caused by the operator not being in the search_path. note that the casts in the columns list are still needed, as described in the associated comment.

schemas/util.sql: materialize_query(): add a comment on the table with the query it was generated from

schemas/util.sql: debug_print_sql(): util.mk_set_search_path(): use for_printing := true to comment out LOCAL

schemas/util.sql: mk_set_search_path(text): include LOCAL commented out, because the user might want to run it with another statement as a single command, in which case it would be useful because they will be in the same transaction (http://www.postgresql.org/docs/9.3/static/sql-set.html#AEN81154)

schemas/util.sql: mk_set_search_path(): added for_printing option like for mk_set_search_path(text)

schemas/util.sql: mk_set_search_path(): support creating a SET search_path statement for display as well, which would exclude LOCAL because it doesn't work as a standalone command (http://www.postgresql.org/docs/9.3/static/sql-set.html#AEN81154)

schemas/util.sql: debug_print_sql(): include the search_path in case the query contains search_path-dependent elements (such as operators)

schemas/util.sql: util.explain2notice_msg(): add newline before and after to visually separate it from other debug info