Drop Experimental.Impact, simplify related code and tests

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

compiler/testData/cli/jvm/experimentalNested.kt

@@ -1,7 +1,7 @@
 package org.test
 
 class Outer {
-    @Experimental(Experimental.Level.ERROR, [Experimental.Impact.LINKAGE])
+    @Experimental
     annotation class Nested
 }