Several changes.

Add a "once" function for doing evaluation at most once, on demand.

Distinguish between C and null.

Add is_list function.

Add is_good function (opposite of is_void).

Rename Qis_atom is is_type, and flip arguments.

Rename type_eval as type_eager, and rename Qeval as eager.

Rename Qcons as cons.

Clean up comments in format.c and num.c.

Rename curr_name as cur_name in standard.c.

Reorder some matches in standard.c to alphabetical order.

Added tests for new functions in test/a1.fxl.

No need for a separate token_cons in type_resolve.c.  Instead, just use cons.

In the show function for testing, I made the unknown type case print the
internal address of the type function, instead of "_".