schemas/functions.sql: Added _merge relational function

schemas/functions.sql: Added join_strs() aggregate

sql.py: Renamed index_pkey() to add_pkey() to be consistent with add_index()

sql.py: into_table_name(): In function args, omit column name for function result columns

sql.py: into_table_name(): In function args, keep the input table name for input columns to identify where they came from, except for the main input table name because it makes the string too long

sql_gen.py: esc_name(): Don't return plain name if is_safe_name(), because this makes the SQL inconsistent when some names have "_"s and some don't

sql.py: index_pkey(): Use sql_gen.add_suffix() to ensure index name isn't too long

sql.py: put_table(): insert_out_pkeys, insert_in_pkeys: Use sql_gen.add_suffix() to ensure name isn't too long

sql.py: next_version(): Use new sql_gen.add_suffix(). Removed identifier_max_len because it is now in sql_gen.

sql_gen.py: Added identifier_max_len and add_suffix()