schemas/functions.sql: _nullIf(): Fixed bug where wrong var name was used to retrieve type param. Reenabled _nullIf now that it's working.

sql_gen.py: ColDict.__setitem__(): Translate NULL values to the appropriate default value of the key column (which is often NULL, but not always) so that select query filtering/joins work correctly

sql.py: DbConn.col_default(): Fixed bug where returned string needed to be wrapped in sql_gen.as_Code() because it's sometimes a SQL expression and sometimes NULL

sql_gen.py: Added as_Code(). Split SQL code objects into separate sections so unparameterized classes would be separate from general classes.

sql_gen.py: Code.to_str(): Fixed bug where needed to raise NotImplementedError, not NotImplemented

dicts.py: is_dict(): Fixed bug where lists also have getitem() methods, so keys() was checked for instead

sql.py: put_table(): Fixed bug where mapping was still a plain dict because that's what dicts.join() returns, by moving the wrapping of it in a sql_gen.ColDict after dicts.join()

sql_gen.py: ColDict: Fixed bug where copy() needed to be implemented

sql.py: into_table_name(): Fixed bug where key needed to be passed through str() now that ColDict uses Col objects for everything

sql_gen.py: ColDict: Extend UserDict.DictMixin instead of dict because its non-core function implementations route all inner dict accesses to the core functions getitem() and setitem(). sql.py: put(): DuplicateKeyException: Wrap util.dict_subset_right_join() in a sql_gen.ColDict because the dict returned by util.dict_subset_right_join() is just a plain dict. (This change must happen at the same time because the previous functionality relied on a bug in ColDict.)