Primitives now have a signature of prim(ctx, self, n, args ...)
instead of just prim(ctx, args ...). This allows for variadic
primitives, should help clean up exception handling, and will
allow primitives generated from Scheme->C or JIT compilation.
The primitives sometimes used as utility functions from C such as
sexp_memq have been renamed with a "_op" suffix (e.g. sexp_memq_op)
and a macro sexp_memq has been provided filling in the self and n
args automatically. The self is passed as NULL in these macros,
but will be probably replaced with a reference to the opcode later.
