sql.py: mk_select(): joins: Fixed bug where USING-based joins did not have closing ")"

db_xml.py: put_table(): Fixed bug where in_table was last in in_tables instead of first, causing it to be ignored by the current put_table() implementation, which only considers the first table name

db_xml.py: put_table(): Fixed bug where pkeys_table returned by recursive call to put_table() needed to be prefixed with $ to be treated as an input column name rather than a literal value

sql.py: mk_select(): Support joins with USING, which can be used to merge multiple input cols into the same output col

sql.py: mk_insert_select(): embeddable: Fixed bug where query that uses function was being sorted by its first column (the default mk_select() setting), when it should be left in its original order

sql.py: put_table(): Take a dict mapping out to in cols instead of separate in and out cols lists

sql.py: mk_select(): Joins: Reversed order of left_col and right_col in the joins dict as well, so the joined table's columns are the keys

sql.py: mk_select(): Joins: Reversed order of left_col and right_col so the column of the table being joined is first, to match the form of a WHERE clause

sql.py: mk_select(): Support joins

sql.py: mk_select(): Accept a list of tables to join together (initial implementation just uses the first table)