Replace invalid Map.t type with map type

[coryc5]

Hello and thanks for your work on this library.

My team is using this library along with dialyzer, and we encountered this error after adding new-relixir:

:0: Unknown type 'Elixir.Map':t/0

I am making this PR to replace the Map.t type with the map type.

[coryc5]

Looks like there are some race conditions within the tests, they alternate between passing/failing for me locally

[coburncoburn]

thanks!