Support signatures in DER format

[fhunleth]

Currently signatures are returned as raw bits. It's also common to need DER-encoded signatures. These are a few bytes longer, and it can be confusing since other APIs often assume one or the other without going into the details. It would be nice to have better documentation in this area and a helper function to return the DER-encoded version.

Here's some code from https://gist.githubusercontent.com/voltone/d3c0bb3ee821703f52d439e00262cb88/raw/e90117c60dceb07f3a674cbf5102ef6f2722e0c8/ecdsa_signature.ex:

defmodule ECDSASignature do
  require Record

  Record.defrecord(
    :ecdsa_signature,
    :"ECDSA-Sig-Value",
    Record.extract(:"ECDSA-Sig-Value", from_lib: "public_key/include/OTP-PUB-KEY.hrl")
  )

  def new(r, s) when is_integer(r) and is_integer(s) do
    ecdsa_signature(r: r, s: s)
  end

  def new(raw) when is_binary(raw) do
    size = raw |> byte_size() |> div(2)
    <<r::size(size)-unit(8), s::size(size)-unit(8)>> = raw
    new(r, s)
  end

  # Export to DER binary format, for use with :public_key.verify/4
  def to_der(ecdsa_signature() = signature) do
    :public_key.der_encode(:"ECDSA-Sig-Value", signature)
  end
end