Add GenServer Elixir module

[bettio]

A limited number of function of this module should be chosen and implemented. An issue for each group of related functions should be opened.

[petermm]

A gist for GenServer.ex is here https://gist.github.com/petermm/1c5eda55bbd024c4f1f331e23222b360 pending https://github.com/atomvm/AtomVM/pull/1176

copy/pasted from Elixir source and adapted. will document steps better, out-commented code left in there intentionally..

1. :gen module is not available, so calling :gen_server instead
2. no distributed erlang

more than likely something missing and or completely wrong at the moment.. will look into getting a test suite running.

Tested successfully by running https://github.com/elixir-circuits/circuits_i2c on device.