New script to run cargo-semver-checks with appropriate options.

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.