libsecp256k1 0.7.1 released

We'd like to announce the release of version 0.7.1 of libsecp256k1: https://github.com/bitcoin-core/secp256k1/releases/tag/v0.7.1

This is a maintenance release with a minor security improvement, a new test framework, and some build system changes.

For the full changelog, see https://github.com/bitcoin-core/secp256k1/blob/master/CHANGELOG.md