The options are rather complicated; because we do not want to
subject our experimental features to semver, we need to run generate
JSON rustdoc on our own and then pass that JSON to
cargo-semver-checks. This in turn requires us to use the same
options that cargo-semver-checks uses, including "RUSTC_BOOTSTRAP".
I've left some TODOs here in places where we will likely want to
improve our code in the future.
See #711.
Nick Mathewson