Publish crate to crates.io

Upgrade dependencies and use a Git submodule for libsecp256k1

[Cargo.toml] update rand to 0.7 (breaking change)

Also add Parity as an author.

ci: add .gitlab-ci.yml

* master:
  Update src/ffi.rs
  s/EcdhHashFunction/EcdhHashFn
  revert unrelated change
  refactor ecdh_raw
  fix ecdh ffi
  fix rand tests
  remove schnorr support

fix ecdh ffi

Co-Authored-By: David <dvdplm@gmail.com>

update c-lib to upstream

Also update build.rs and ext.c

[Cargo.toml] update rand to 0.6 (breaking change)

Reexport the rand crate

Remove ! from first link, 

Unbreak build on OpenBSD and other OS that aren't linux, windows, or …