Implement `replace_null()`

[markfairbanks]

library(tidyverse)

df <- tibble(x = c(1, NA))

df %>%
  mutate(x = replace_na(x, 2))
#> # A tibble: 2 × 1
#>       x
#>   <dbl>
#> 1     1
#> 2     2

df %>%
  replace_na(list(x = 2))
#> # A tibble: 2 × 1
#>       x
#>   <dbl>
#> 1     1
#> 2     2