Upgrade libsecp256k1 dependency and disable bindgen in default-features

[xoloki]

As seen in #81, bindgen does not handle the current libsecp256k1 particularly well. This is largely the fault of the layered #defines that now are used in the internal headers.

So we will need to disable bindgen by default for now, in order to accomplish the necessary goal of upgrading a 19 month old libsecp256k1.