Since we're not yet sure of the design of Experimental/UseExperimental,
we're making them "experimental" themselves in some sense, in that the
user is required to provide the magic argument
"-Xuse-experimental=kotlin.Experimental" to be allowed to use either
Experimental or UseExperimental. This is more convenient than the
previous approach of "-language-version 1.3
-Xskip-metadata-version-check" because it's simpler and does not cause
pre-release binaries to be produced
See https://github.com/Kotlin/KEEP/issues/95#issuecomment-383889404
Drop Experimental.changesMayBreak, Experimental.Impact, the concept of
signature/body usage, same module exemption. Make the majority of tests
single-module because there is now no difference in the checker between
usages from the same module or from another module
This results in more diagnostics usually, but allows library authors to
avoid annotating everything in each experimental class with the marker
(only the class needs to be annotated now)
#KT-22759
Alexander Udalov