Update bindings to current secp256k1 library

rust-secp256k1 was based off of https://github.com/sipa/secp256k1,
which has been inactive nearly as long as this repository (prior to
a couple days ago anyway). The correct repository is

   https://github.com/bitcoin/secp256k1

This is a major breaking change to the library for one reason: there
are no longer any Nonce types in the safe interface. The signing functions
do not take a nonce; this is generated internally.

This also means that I was able to drop all my RFC6979 code, since
libsecp256k1 has its own implementation.

If you need to generate your own nonces, you need to create an unsafe
function of type `ffi::NonceFn`, then pass it to the appropriate
functions in the `ffi` module. There is no safe interface for doing
this, deliberately: there is basically no need to directly fiddle
with nonces ever.