fully qualify Witchcraft.Apply.then/2 to avoid name conflict in Elixir 1.12+

[baseballlover723]

Summary

Elixir 1.12 introduces a new kernel method: Kernel.then/2, this causes witchcraft to not compile on Elixir 1.12+ with

** (CompileError) lib/witchcraft/foldable.ex:751: function then/2 imported from both Witchcraft.Apply and Kernel, call is ambiguous
    (elixir 1.12.0-rc.1) src/elixir_dispatch.erl:42: :elixir_dispatch.import_function/4
    (elixir 1.12.0-rc.1) src/elixir_fn.erl:76: :elixir_fn.capture_import/4
    (stdlib 3.14.2) lists.erl:1358: :lists.mapfoldl/3
    (stdlib 3.14.2) lists.erl:1359: :lists.mapfoldl/3

This PR fully qualifies Witchcraft.Apply.then/2 so that then/2 is no longer ambiguous.

This PR fixes/implements the following bugs/features

• [ ] Support Elixir 1.12+

Explain the motivation for making this change. What existing problem does the pull request solve? You can't use witchcraft on Elixir 1.12+.

Test plan (required)

Run the tests with Elixir 1.12.2

Closing issues

Fixes #

After Merge

• [ ] Does this change invalidate any docs or tutorials? If so ensure the changes needed are either made or recorded
• [ ] Does this change require a release to be made? Is so please create and deploy the release