Implement basic - and *

Although multiplication doesn't work, because the `mul` instruction
seems to be a bit special.

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))
    ((-)
     (emit-expr (primcall-operand2 x) si env)
     (emit "movl %eax, ~a(%rsp)" si) ; move second arg on the stack
     (emit-expr (primcall-operand1 x) (- si wordsize) env)
     (emit "subl ~a(%rsp), %eax" si))
    ((*)
     (emit-expr (primcall-operand2 x) si env)
     (emit "movl %eax, ~a(%rsp)" si) ; move second arg on the stack
     (emit-expr (primcall-operand1 x) (- si wordsize) env)
     (emit "mull ~a(%rsp), %eax" si))
    ((cons)
     (emit-expr (primcall-operand1 x) si env)
     (emit "movl %eax, 0(%rsi)") ; set the car

scm/inc/tests.scm

    ["(+ 0 0)" "0\n"]
    ["(+ 41 1)" "42\n"]
    ["(+ 1 41)" "42\n"]
    ["(+ -10 11)" "1\n"]])
    ["(+ -10 11)" "1\n"]
    ["(- 1 2)" "-1\n"]
    ["(- 42 0)" "42\n"]
    ["(- 0 42)" "-42\n"]
    ;["(* 3 4)" "12\n"]
    ])
(do-tests "3.5 - local variables"
  '[["(let ((x 3) (y 4)) (+ x y))" "7\n"]