Fix #1208: toString for Semantics and CheckedBehaviors

Looks like this:

    Semantics(
      CheckedBehaviors(
        asInstanceOfs = Unchecked
      ),
      strictFloats = false
    )
Fix #1212: Run CI against Java 1.{6-8}
Merge pull request #1220 from gzm0/sem-tostring

Fix #1208: toString for Semantics and CheckedBehaviors
Buglist partests arrayclone-{old,new} and issue192.

They're broken on master.
Merge pull request #1221 from gzm0/cross-jdk

Fix #1212: Run CI against Java 1.{6-8}
Merge pull request #1223 from sjrd/buglist-broken-partests

Buglist partests arrayclone-{old,new} and issue192.
Fix #1222: The optimizer breaks Array.clone().
Blacklist run/issue192.scala because it relies on strict floats.
Merge pull request #1225 from sjrd/fix-reachability-array-clone

Fix broken partests