expt must pop stack

[dcurrie]

The expt function does not manage the stack correctly. This fixes it for me:

static void primop_expt(long arg) {
	double tmp = the_double(1,sp[0]);
	tmp = pow(tmp,the_double(2,sp[1]));
	*++sp = make_flonum(tmp);
}

Note the addition of ++.

[dcurrie]

Test case at prompt:

(- (expt 2 29) 1)

Gives 536870883 but should be 536870911.