`Keyword.from_enum/1` is deprecated. Replace with `Enum.to_list/1`

Running relex on Elixir 0.13.1 fails, as `Keyword.from_enum/1` has been removed.

In Elixir 0.13.0, the correct replacement is `Enum/into/2`, but this isn't available in earlier versions, and would break users on ~ 0.12

Under the hood, `Keyword.from_enum/1` used to just call `Enum.to_list/1`, so this preserves the behaviour. This works on ~> 0.13.0 and ~> 0.12.4