secp256k1: move to top-level packages

Reason: secp256k1 is not a Python package.

pkgs/secp256k1/default.nix

@@ -0,0 +1,28 @@
+{ stdenv, fetchFromGitHub, autoreconfHook }:
+
+let inherit (stdenv.lib) optionals; in
+
+stdenv.mkDerivation {
+  pname = "secp256k1";
+
+  version = "2019-10-11";
+
+  src = fetchFromGitHub {
+    owner = "bitcoin-core";
+    repo = "secp256k1";
+    rev = "0d9540b13ffcd7cd44cc361b8744b93d88aa76ba";
+    sha256 = "05zwhv8ffzrfdzqbsb4zm4kjdbjxqy5jh9r83fic0qpk2mkvc2i2";
+  };
+
+  nativeBuildInputs = [ autoreconfHook ];
+
+  configureFlags = ["--enable-module-recovery" "--disable-jni" "--enable-experimental" "--enable-module-ecdh" "--enable-benchmark=no" ];
+
+  meta = with stdenv.lib; {
+    description = "Optimized C library for EC operations on curve secp256k1";
+    homepage = "https://github.com/bitcoin-core/secp256k1";
+    license = with licenses; [ mit ];
+    maintainers = with maintainers; [ nixbitcoin ];
+    platforms = with platforms; unix;
+  };
+}