get_unique_name() in node_class.R might not be unique

Perhaps this is not likely to happen, or for this to be an issue, but it seems that the rhex() function as defined isn't gauranteed to create a unique name if there are many many nodes (like 1 million).

This is used in [node_class.R](https://github.com/greta-dev/greta/blob/a07d84fc53f71e87790999c1d8da92ac99ed8659/R/node_class.R#L273).

See example below.

``` r
n_rhex <- 1e6

# generate a random 8-digit hexadecimal string
rhex <- function() paste(as.raw(sample.int(256L, 4, TRUE) - 1L), collapse = "")

many_rhex <- replicate(n = n_rhex, expr = rhex(), simplify = "vector")

dplyr::n_distinct(many_rhex)
#> [1] 999874

dplyr::n_distinct(many_rhex) == n_rhex
#> [1] FALSE
```

^{Created on 2021-04-08 by the [reprex package](https://reprex.tidyverse.org) (v2.0.0)}

Perhaps digest or something like https://github.com/coolbutuseless/xxhashlite could be used to give nodes unique IDs