sql.py: put_table(): col_ustr(): Removed no longer needed sql_gen.as_Col() because mapping and join_cols now ensure that their contents are sql_gen.Col objects

schemas/functions.sql: Added _alt relational function

sql.py: put_table(): Make mapping and join_cols a sql_gen.ColDict so that literal values will always be turned into sql_gen.Col objects. DuplicateKeyException: Use dict_subset_right_join() instead of dict_subset() so that all columns in a constraint are included in joins on out_table (such as for a relational function with omitted arguments).

sql_gen.py: Added ColDict

sql_gen.py: as_Col(): Added optional name param to specify that non-Col input will be renamed using NamedCol with the given name

sql.py: put_table(): FunctionValueException: Fixed bug where only function calls, not plain columns, were handled, by using sql_gen.unwrap_func_call() to remove any function call only if there was one

sql_gen.py: Added unwrap_func_call()

bin/map: by_col: Stripping XML functions not in the DB: Fixed bug where preserve_funcs.add() was used when `preserve_funcs |=` should have been used to add the entire iterable that sql.tables() returns

sql.py: not_null_col: Changed value to 'not_null_col' so that column doesn't seem like a status indicator of whether some value is not null (in fact it's just a column that is always not null)

xml_func.py: Replaced xpath.get_1() with xpath.get_value() where possible, for simplicity