We don't use nightly Rust anymore, nor do we try to land docs for
features while they're still unstable, so this check is no longer
needed. The tests will just fail if we try to use a feature flag in code
listings.
We need to track Rust master, but don't want to cover unstable things.
As such, a new lint, `stable-check`, is added.
The second edition was already lint clean, but the first needed some
tweaks:
1. The closures chapter was showing off details of the closure traits,
so just don't do that.
2. The FFI chapter was using internal libc to compile the tests, so just
don't compile them anymore.
3. The compiler plugins chapter was vestigial and was moved to the
unstable book.
Fixes#558
