Add BCV and enable Explicit API mode

[Goooler]

https://github.com/Kotlin/binary-compatibility-validator
https://github.com/Kotlin/KEEP/blob/master/proposals/explicit-api-mode.md

[kizitonwose]

Was a bit worried that I'd have to rebase the multiplatform branch which is under development after this is merged, then I saw that the PR is actually targeting the multiplatform branch, such a relief! Many thanks to you!