(WIP) Drop Erlang support

Warning: This could be controversial!

I’m trying to make a distillery release of a project which includes libsecp256k1. For some reason libsecp256k1_nif.so wasn’t being included in the release on linux and I wanted to eliminate possible variables. This didn’t fix that problem.

I’ve also noticed the following warnings when building libsecp256k1:

warning: variable "description" does not exist and is being expanded to "description()", please use parentheses to remove the ambiguity or change the variable name
  /Users/masonf/src/libsecp256k1/deps/mix_erlang_tasks/mix.exs:9

warning: variable "package" does not exist and is being expanded to "package()", please use parentheses to remove the ambiguity or change the variable name
  /Users/masonf/src/libsecp256k1/deps/mix_erlang_tasks/mix.exs:10

==> mix_erlang_tasks
Compiling 4 files (.ex)
warning: String.to_char_list/1 is deprecated. Use String.to_charlist/1 instead
Found at 4 locations:
  lib/mix/tasks/ct.ex:31
  lib/mix/tasks/ct.ex:32
  lib/util.ex:13
  lib/util.ex:14

Generated mix_erlang_tasks app

This PR did fix those warnings.

Here are the pros and cons I see of merging this:

Pros:

No more build warnings
Removes a dependency on mix_erlang_tasks which hasn’t been updated since 2015
It’s more clear which SHA of the source code we’re on since its right in mix.exs now

Cons:

No more erlang support
Possible unknown breakages (although all the tests pass so I think we should be good there)

Thoughts? cc: @hayesgm @paulperegud @ayrat555

exthereum / libsecp256k1

(WIP) Drop Erlang support #15