Allocate pairs on the heap

Which means there's a heap now.  The pairs can't be printed yet, because
the display thing has to rewritten for that.  (I think there's going to
be a recursive print_value function.)

scm/inc/compiler.scm

     (emit "movl %eax, ~a(%rsp)" si) ; move second arg on the stack
     (emit-expr (primcall-operand1 x) (- si wordsize) env)
     (emit "addl ~a(%rsp), %eax" si))
    ((cons)
     (emit-expr (primcall-operand1 x) si env)
     (emit "movl %eax, 0(%rsi)") ; set the car
     (emit-expr (primcall-operand2 x) si env)
     (emit "movl %eax, 4(%rsi)") ; set the cdr
     (emit "movq %rsi, %rax") ; rax = rsi | 1  (cons cell/pair tag)
     (emit "orq  $~a, %rax" #b001)
     (emit "addq $8,  %rsi")) ; bump rsi
    ))
(define (compile-program x)
  (display ".globl scheme_entry\n\n")
  (display "scheme_entry:\n")
  (emit "movq %rax, %rsi") ; store pointer to heap memory
  (emit-expr x (- wordsize) '())
  (emit "ret"))

scm/inc/driver.c

/* a simple driver for scheme_entry */
#include <stdio.h>
#include <stdlib.h>
#define fixnum_mask  3 // 11
#define fixnum_tag   0 // 00
#define empty_list   47 // 00101111
int scheme_entry();
#define cons_tag  1 // 001
#define cons_mask 7 // 111
#define MAX_MEMORY (1 << 20)
int scheme_entry(void *memory);
int main(int argc, char **argv) {
	int val = scheme_entry();
	void *mem = malloc(MAX_MEMORY);
	if (mem == NULL) {
		perror("malloc");
		exit(1);
	}
	int val = scheme_entry(mem);
	if ((val & fixnum_mask) == fixnum_tag) {
		printf("%d\n", val >> fixnum_shift);
	} else if ((val & char_mask) == char_tag) {
		printf("#\\%c\n", val >> char_shift);
	} else if ((val & boolean_mask) == boolean_tag) {
		printf("#%s\n", (val >> boolean_shift) == 1 ? "t" : "f");
	} else if ((val & cons_mask) == cons_tag) {
		printf("cons!\n");
	} else if (val == empty_list) {
		printf("()\n");
	} else {

scm/inc/program.scm

(load "compiler.scm")
(compile-program '(let ((x 3) (y 4)) (if (integer? (+ x y)) 42 #f)))
(compile-program '(cons 10 20))