Some changes to make calling primitives from compiled code cheaper:

Some changes to make calling primitives from compiled code cheaper:

- Primitives back out in the interpreter, rather than directly.
- A table with the arity in bytes has been added to usrdef.c
- Primitive objects have two fields: a table index for invocation,
and a virtual number for bookkeeping purposes.  The table index is
always valid, even for unimplemented primitives.  In this case it
points to a procedure which causes an UNIMPLEMENTE-PRIMITIVE error
when invoked.  The back out mechanism takes care