Upgrade the vendored `libsecp256k1` code to v0.2.0#567

Merged

apoelstra merged 2 commits intorust-bitcoin:masterfrom tcharding:12-16-secp-sys-0.2.0

Dec 21, 2022

+9,140-6,757

Commits on Dec 19, 2022

Update scratch_impl.h patch file
tcharding
committed

Commits on Dec 20, 2022

Upgrade the vendored libsecp256k1 code
tcharding
committed