sql_gen.py: Join.to_str(): Fixed bugs revealed in first test of function

db_xml.py: put_table(): Turn off table_is_esc when calling sql.put_table() and don't escape out_table

sql.py: mk_insert_select(): Use sql_gen.table2sql_gen().to_str() to escape the table

db_xml.py: put_table(): First in_tables table is sql_gen.Table object

db_xml.py: put_table(): Converted row (mapping) values to sql_gen objects

sql.py: mk_select(): Accept main tables (table0's) that are Table objects. This change requires plain SQL code to be wrapped in a CustomCode object if it should not be unescaped and converted to a Table object.

sql_gen.py: as_Table(): Accept tables that are Code objects, not just Table objects

sql_gen.py: CustomCode: Fixed bug where needed to inherit from Code

sql.py: put_table(): Return a sql_gen.Col object instead of an old-style tuple

sql.py: mk_select(): joins: Switched to using filter_out as an attribute of the Join object instead of a sentinel value for the first column. Filter by the right table's pkey being NULL instead of each joined column being NULL, because some joined columns may contain NULL values which would mess things up, but the pkey presumably is NOT NULL.