Elixir 1.18 compatibility

[maennchen]

When using excoveralls with elixir main, the following error is produced:

     ** (Protocol.UndefinedError) protocol Enumerable not implemented for {"defmodule Test do", nil} of type Tuple. This protocol is implemented for the following type(s): Date.Range, File.Stream, Function, GenEvent.Stream, HashDict, HashSet, IO.Stream, List, Map, MapSet, Range, Stream

This is because we provide a list of tuples, but that is not valid according to the typespec @spec zip([list()]) :: [tuple()].

Since Elixir is deprecating List.zip/1, the code was slightly changed and a list of tuples is no longer accepted.

This PR switches to Enum.zip/1, and uses a list of lists instead of tuples. That should be compatible since Elixir 1.4 and avoid upcoming deprecations.

[maennchen]

I've reported the issue upstream and the change that leads to the error has been reverted: elixir-lang/elixir@c60b477

It's however still not complying with the spec and using a deprecated function. Therefore the PR is still relevant.

[parroty]

Thank you!