schemas/functions.sql: Added plain function _merge() and use it in trigger function _merge()

schemas/functions.sql: Added plain function _alt() and use it in trigger function _alt()

schemas/functions.sql: Removed no longer used ensure_not_null()

sql.py: put_table(): MissingCastException: Use cast_temp_col() so that cast will occur before any main insert, which locks the output table and should take as little time as possible

sql.py: Added cast_temp_col()

sql.py: add_col(): Support additional run_query() kw_args. add_row_num(): Use new add_col().

sql.py: Added add_col()

sql_gen.py: Col.__str__(): Truncate any table name using concat() to ensure that the full column name is included in the string

strings.py, sql_gen.py: Renamed add_suffix() to concat() to reflect that this is a fixed-length replacement for +

sql.py: put_table(): Moved MissingCastException to the top of the exceptions list because it's more of a core exception than the others, and will be raised before any rows are even inserted