Remove KnownDomain preparation

[leonschoorl]

In preparation for a 1.8 release before merging #2589, this contains the non backwards compatible API changes of #2589.

And some minor cleanup and improvements (Cleanup clash-lib.cabal and Improve blackbox error message)

Still TODO:

• [x] Write a changelog entry (see changelog/README.md)
• [x] Check copyright notices are up to date in edited files