We are upgrading the MSRV across the whole Rust Bitcoin ecosystem.
Update the README, clippy config file, and CI to use the new MSRV.
Changes to use the new MSRV will be done later.
Add mention of MSRV to `secp256k1-sys`, add unreleased section to both
changelogs.
This feature was not useful for Cargo users, since Cargo does not give you
the kind of fine-grained control over C library linkage that you need. So
it was just unnecessarily confusing and would cause the build to break if
you enabled it accidentally, say, with --all-features.
Tobin C. Harding
Andrew Poelstra
Steven Roose