Introduce tagged literals/immediate values

We don't test values other than integers yet, but it seems to work for
those.  At least the test program prints 42 as before.

scm/inc/compiler.scm

@@ -3,11 +3,26 @@
  (display (apply format instr args))
  (display "\n"))
(define fixnum-shift 2)
(define char-shift 8)
(define boolean-shift 7)
(define (compile-program x)
  (define (immediate-rep x)
    (cond
      ((integer? x) (bitwise-arithmetic-shift x fixnum-shift))
      ((char? x) (bitwise-xor (bitwise-arithmetic-shift (char->integer x) 8) char-shift))
      ((boolean? x) (bitwise-xor (bitwise-arithmetic-shift (cond
                                                             ((boolean=? x #t) 1)
                                                             ((boolean=? x #f) 0))
                                                           boolean-shift)
                                 #b0011111))
      ((eq? x '()) #b00101111)))
  (display ".globl scheme_entry\n\n")
  (display "scheme_entry:\n")
  (emit "movl $~a, %eax" x)
  (emit "movl $~a, %eax" (immediate-rep x))
  (emit "ret"))
(compile-program 42)

scm/inc/driver.c

@@ -2,9 +2,19 @@
#include <stdio.h>
#define fixnum_mask  3 // 11
#define fixnum_tag   0 // 00
#define fixnum_shift 2
int scheme_entry();
int main(int argc, char **argv) {
	printf("%d\n", scheme_entry());
	int val = scheme_entry();
	if ((val & fixnum_mask) == fixnum_tag) {
		printf("%d\n", val >> fixnum_shift);
	} else {
		printf("Error: unhandled value: %d\n", val);
		return 1;
	}
	return 0;
}