Another spot to fully cross version continuations plugin

The continuations plugin is fully cross versioned, as it
dependes on the non-BC compiler API.

I had to make the same change externally releasing 2.11.0:

   https://github.com/scala/jenkins-scripts/pull/99/files

This fix addresses the problem in scala/scala. Without it,
we can't resolve dependencies in development.
Merge pull request #3688 from retronym/topic/continuations-cross-version-strikes-back

Another spot to fully cross version continuations plugin
WIP: trying to apply the Literal-based singleton types commit by @paulp to the master.
Fix non-working things.
Update check file.
Problems with null.
Update the check file.
This is needed for the tests to pass.
Added a macro to materialize an inhabitant for the singleton type.
Added a test with the Ranged class mentioned by @non.
Add inhabitant to the bincompat whitelist.
Marked potentiall places in code where constant type folding and singleton type inlining might occur.
Inlining functions with singleton result type.
More involved example with Ranged.
Needed for the tests to pass after adding the SingleInhabitant
Checking for declared singleton type. Tests seem to pass, but the issue is still there.
Forward bincompat.
Another problem.
Check that parser allows unicode identifiers in the type position.
Another one