Allow attributes on a lazyvec

Currently, setting an attribute to an ALTREP vector, triggers a copy and after that the vector is no longer an ALTREP:

``` r
x <- 1:10
lazyvec::is_altrep(x)
#> [1] TRUE

class(x) <- "my_class"
lazyvec::is_altrep(x)
#> [1] FALSE
```

Perhaps a custom _lazyvec_ can store attributes inside the payload object.