Add check_no_api_breakages, a script to check public API breakages (#1016)
Add check_no_api_breakages, a script to check public API breakages (#1016)

Motivation:

It's incredibly hard not to accidentally break the public API, so let's
automate that :).

Modifications:

Add the first version of a public API check script.

Result:

Fewer public API breakages.
apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio