get rid of dynamic code generation

[ayrat555]

There are a couple of problems with current implementation:

1. Signature for api methods is ugly and it's not apparent what parameters every method accepts.

  # map in list
  HttpClient.eth_send_transaction([%{...}]) 

  # list of binaries
  HttpClient.eth_get_storage_at([binary1, binary2, binary3])

2. It's hard to mock this methods for tests because all methods are dynamically generated by macro.

The solution is to define explicit behaviour with all api methods and then implement this behavour in all client modules.

defmodule Ethereumex.Client.Behaviour do
  @type return_type :: {:ok, map()} | {:error, map()} | {:error, atom()}

  # API methods

  @callback web3_client_version() :: return_type
  @callback web3_sha3(binary()) :: return_type
  @callback net_version() :: return_type
  @callback net_peer_count() :: return_type
  ...

  # actual request

  @callback request(map()) :: return_type

dashbitco/mox#13