Change APPEND opcode to directly modify a variable.

Avoids a big O(n^2) loop in constructing arrays. Fixes #61.

compile.c

@@ -292,11 +292,7 @@ block gen_collect(block expr) {
                                gen_noop(), OP_HAS_VARIABLE);
   block c = BLOCK(gen_op_simple(DUP), gen_const(jv_array()), array_var);
 
-  block tail = BLOCK(gen_op_simple(DUP),
-                     gen_op_var_bound(LOADV, array_var),
-                     gen_op_simple(SWAP),
-                     gen_op_simple(APPEND),
-                     gen_op_var_bound(STOREV, array_var),
+  block tail = BLOCK(gen_op_var_bound(APPEND, array_var),
                      gen_op_simple(BACKTRACK));
 
   return BLOCK(c,

execute.c

@@ -236,9 +236,12 @@ jv jq_next() {
     case APPEND: {
       // FIXME paths
       jv v = stack_pop().value;
-      jv array = stack_pop().value;
-      array = jv_array_append(array, v);
-      stack_push(stackval_root(array));
+      uint16_t level = *pc++;
+      uint16_t vidx = *pc++;
+      frame_ptr fp = frame_get_level(&frame_stk, frame_current(&frame_stk), level);
+      jv* var = frame_local_var(fp, vidx);
+      assert(jv_get_kind(*var) == JV_KIND_ARRAY);
+      *var = jv_array_append(*var, v);
       break;
     }
 

opcode_list.h

@@ -12,7 +12,7 @@ OP(FORK,  BRANCH,   0, 0)
 OP(JUMP,  BRANCH,   0, 0)
 OP(JUMP_F,BRANCH,   1, 0)
 OP(BACKTRACK, NONE, 0, 0)
-OP(APPEND, NONE,    2, 1)
+OP(APPEND, VARIABLE,1, 0)
 OP(INSERT, NONE,    4, 2)
 
 OP(GETPATH, NONE,   2, 1)