Fix dyalizer issues (#355)

Fix several dialyzer issues. Some simply break contract because they
allow a `nil`, while others were slightly more complicated (like the
supertype being a `struct()` instead of a `%{}`).

apps/ex_wire/lib/ex_wire/framing/frame.ex

@@ -183,7 +183,7 @@ defmodule ExWire.Framing.Frame do
           MAC.mac_inst(),
           ExthCrypto.Cipher.cipher(),
           ExthCrypto.Key.symmetric_key(),
-          binary()
+          binary() | nil
         ) :: MAC.mac_inst()
   defp update_mac(mac, mac_encoder, mac_secret, seed) do
     final = mac |> MAC.final() |> Binary.take(16)

apps/ex_wire/lib/ex_wire/handshake/struct/auth_msg_v4.ex

@@ -33,7 +33,7 @@ defmodule ExWire.Handshake.Struct.AuthMsgV4 do
     ]
   end
 
-  @spec deserialize(ExRLP.t()) :: t
+  @spec deserialize(nonempty_maybe_improper_list()) :: t
   def deserialize(rlp) do
     [signature | rlp_tail] = rlp
     [initiator_public_key | rlp_tail] = rlp_tail

apps/ex_wire/lib/ex_wire/packet.ex

@@ -7,7 +7,7 @@ defmodule ExWire.Packet do
 
   alias ExWire.Packet
 
-  @type packet :: %{}
+  @type packet :: struct()
   @type block_identifier :: binary() | integer()
   @type block_hash :: {binary(), integer()}
 

apps/ex_wire/lib/ex_wire/packet/transactions.ex

@@ -78,9 +78,8 @@ defmodule ExWire.Packet.Transactions do
       ...> |> ExWire.Packet.Transactions.handle()
       :ok
   """
-  @spec handle(ExWire.Packet.packet()) :: ExWire.Packet.handle_response()
+  @spec handle(ExWire.Packet.packet()) :: :ok
   def handle(packet = %__MODULE__{}) do
-    # TODO: Do.
     Logger.debug(fn -> "[Packet] Peer sent #{Enum.count(packet.transactions)} transaction(s)." end)
 
     :ok

apps/exth_crypto/lib/exth_crypto/mac.ex

@@ -24,7 +24,7 @@ defmodule ExthCrypto.MAC do
       iex> ExthCrypto.MAC.mac("The quick brown fox jumps over the lazy dog", "key", :sha256, 8)
       <<247, 188, 131, 244, 48, 83, 132, 36>>
   """
-  @spec mac(iodata(), iodata(), Hash.hash_algorithm(), integer()) :: mac
+  @spec mac(iodata(), iodata(), Hash.hash_algorithm(), integer() | nil) :: mac
   def mac(data, key, hash_algorithm, length \\ nil) when is_atom(hash_algorithm) do
     if Enum.member?(Hash.hash_algorithms(), hash_algorithm) do
       case length do