[API BREAK] update for libsecp256k1 "explicit context" API break

Rather than have global initialization functions, which required
expensive synchronization on the part of the Rust library,
libsecp256k1 now carries its context in thread-local data which
must be passed to every function.

What this means for the rust-secp256k1 API is:
  - Most functions on `PublicKey` and `SecretKey` now require a
    `Secp256k1` to be given to them.

  - `Secp256k1::verify` and `::verify_raw` now take a `&self`

  - `SecretKey::new` now takes a `Secp256k1` rather than a Rng; a
    future commit will allow specifying the Rng in the `Secp256k1`
    so that functionality is not lost.

  - The FFI functions have all changed to take a context argument

  - `secp256k1::init()` is gone, as is the dependency on std::sync

  - There is a `ffi::Context` type which must be handled carefully
    by anyone using it directly (hopefully nobody :))