Update for 0.2.0 release

[ajtowns]

README.md says "The library bundles its own libsecp256k1 currently, as there is no versioning to allow us to safely determine compatibility with an installed library, especially as we also build all the experimental modules."

But libsecp256k1 now does have versioning as of its 0.2.0 release! Presumably would make sense to update to take advantage of that.

https://github.com/bitcoin-core/secp256k1/releases/tag/v0.2.0
https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2022-December/021271.html

[fametrano]

ACK