Add bytecode VM backend (compile AST to bytecodes + stack-based VM)

New execution mode: ./run vm <file.j> compiles AST to bytecodes and
runs them in a while/switch loop. Ints/floats live on the stack (no
heap allocation), ~7.7x faster than the tree-walking interpreter.

Implements: opcodes, compiler with backpatching (if/while), stack VM
with arithmetic, comparisons, variables, strings, and print/println.
Reorganizes backend into src/backend/eval/ and src/backend/bytecode/.

src/frontend/parser.h

@@ -335,6 +335,9 @@ void ast_print(ASTNode *node, const char *prefix, int is_last) {
     ast_print(node->data.while_loop.cond, new_prefix, 0);
     ast_print(node->data.while_loop.body, new_prefix, 1);
     break;
+  case NODE_NOP:
+    printf("NODE_NOOP\n");
+    break;
 
   case NODE_CALL:
     printf("NODE_CALL(\"%s\")\n", node->data.call.name);