std: fix parse_u64 implementation and add test

also codegen implement comparision with pure errors

src/codegen.cpp

@@ -1269,6 +1269,9 @@ static LLVMValueRef gen_cmp_expr(CodeGen *g, AstNode *node) {
         } else {
             zig_unreachable();
         }
+    } else if (op1_type->id == TypeTableEntryIdPureError) {
+        LLVMIntPredicate pred = cmp_op_to_int_predicate(node->data.bin_op_expr.bin_op, false);
+        return LLVMBuildICmp(g->builder, pred, val1, val2, "");
     } else {
         zig_unreachable();
     }