add support for arithmetic operations

include/picrin.h

@@ -16,7 +16,8 @@ typedef struct {
   pic_value *sp;
   pic_value *stbase, *stend;
 
-  pic_value sDEFINE, sCONS, sADD;
+  pic_value sDEFINE, sCONS;
+  pic_value sADD, sSUB, sMUL, sDIV;
   struct pic_env *global_env;
 
   struct heap_page *heap;

include/picrin/irep.h

@@ -10,6 +10,9 @@ enum pic_instruction {
   OP_CALL,
   OP_CONS,
   OP_ADD,
+  OP_SUB,
+  OP_MUL,
+  OP_DIV,
   OP_STOP
 };