Adds the binary compatibility validator plugin

[Jolanrensen]

Fixes #634 by adding https://github.com/Kotlin/binary-compatibility-validator

See the individual commits for all checks. I started by adding the .api files built using the 0.14.1 release branch, to see whether we already made changes to the API.

This correctly showed me:

• removed deprecated functions in 0.14 post release Part 2 #877, which is fine of course :)
• removed GenericColumn from Revert insertGenericTreeImpl #923, which probably won't cause issues as it was just used from the compiler plugin
• Split openApi module Split OpenAPI in module needed for user projects and module needed for code-generation #916, for which we need to warn users (but that was already obvious)

The plugin automatically runs when check or build runs, so that's nice :) I added it only to the public modules, skipping the examples, plugins, etc.

When updating the public API we'll need to try to keep binary compatibility as best as possible, resorting to deprecating old functions instead of just removing/changing them. And if we introduce a breaking change, we'll need to inform the users in the release notes and run apiDump to overwrite the .api files.

A non-breaking change would be a + in the API check, this means we simply add another function or overload. A breaking change would be a -. Those will cause NoSuchMethodErrors when called from an older version:

For more information, check https://kotlinlang.org/docs/api-guidelines-backward-compatibility.html

Hint: Sometimes DeprecationLevel.HIDDEN can help to remove a function from the API while keeping it in the bytecode. The Api checker plugin still shows it as a -, but it can still be called from a compiled library, avoiding a NoShuchMethodError.

[zaleslaw]

Let's disable for a while

[zaleslaw]

From other hand, it's just a check and could help to discover stuff, which should be documented!

[Jolanrensen]

It doesn't hurt to add it, right? It will just inform you when you make a binary breaking change, after which you can call apiDump, commit the result and the error goes away. But most importantly it makes you aware of the changes that we'll need to inform our users about for the next release.