We are intending to release without having completed our async
signing logic, which sadly means we need to cfg-gate it to ensure
we restore the previous state of panicking on signer errors, rather
than putting us in a stuck state with no way to recover.
Here we add a new `async_signing` cfg flag and use it to gate all
the new logic from #2558 effectively reverting commits
1da29290e7 through
014a336e59.
Rust is fairly relaxed in checking the validity of arguments
passed to #[cfg]. While it should probably be more strict when
checking features, it cannot be strict when checking loose cfg
tags, because those can be anything and are simply passed to rustc
via unconstrained arguments.
Thus, we do it for rustc manually, but scanning all our source and
checking that all our cfg tags match a known cfg tag.
Fixes#2184
