[fexl]

I have decided not to store type pointers directly in the Fexl arena. Instead, I will use a type number, which will be an integer offset into a static array of function pointers in the C program.

That way I could use a memory-mapped array for the arena, making it suitable as a persistent state for a Fexl computation, which could easily be stopped and resumed.

This also avoids the need for a special "show" function pointer in a type structure. Instead, the show functions could be stored in a parallel array of function pointers -- again, indexed by type number.