db_xml.py: put_table(): Recurse into forward pointers

sql.py: put_table(): Take multiple in_tables. Initial implementation just used the first in_table.

sql.py: Added add_row_num(). put_table(): Add row_num to pkeys_table, so it can be joined with in_table's pkeys.

sql.py: Added run_query_into() and use it in insert_select()

sql.py: pkey(): Support escaped table names

sql.py: mk_insert_select(): embeddable: Name the function alias "f" since it will just be wrapped in a nested SELECT, so the exact name doesn't matter (and won't be visible outside the nested SELECT anyway)

db_xml.py: put_table(): Return the (table, col) where the pkeys are made available, now that this information is available from sql.put_table()

sql.py: put_table(): Return just the name of the table where the pkeys are made available, since the column name in that table now equals the pkey name

sql.py: mk_insert_select(): embeddable: Make the column returned by the function have the same name as the returning column

db_xml.py: put_table() Use new sql.put_table()