cargo: whitelist cfg flags for latest versions of cargo

cargo now complains about #[cfg] attributes that don't match cargo
features, unless you explicitly whitelist them. This is very useful for
catching typos. Whitelist the ones we use, and set the lint from "warn"
to "deny".

Author: apoelstra

Cargo.toml

@@ -18,3 +18,6 @@ alloc = []
 
 [target.'cfg(mutate)'.dev-dependencies]
 mutagen = { git = "https://github.com/llogiq/mutagen" }
+
+[lints.rust]
+unexpected_cfgs = { level = "deny", check-cfg = [ 'cfg(bench)', 'cfg(kani)', 'cfg(mutate)' ] }