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

xml_func.py: strip(): Evaluate structural functions like _ignore and _ref by process() instead of removing them. Store structural functions' names in structural_funcs module var. This ensures that _ref targets are still expanded in column-based import.

xpath.py: get(): Create attrs: Put keys last so that any lookahead assertion's path will be created last as it would have without the assertion. This ensures that any value argument of an XML function will always go last even if a lookahead assertion would otherwise have caused it to be created with the element's keys, which previously were created before the attributes.

sql.py: put_table(): If is_func, default into table name ends in () instead of '-pkeys'