Bump version for 2.12.x

For this step, we still depend on _2.11 libraries (partest, xml,
parser-combinators).

After we release the first milestone of 2.12, we will update
versions.property to bootstrap to 2.12 dependencies.
Disable MiMa on the 2.12.x branch
Merge pull request #3715 from retronym/topic/bump

Bump version for 2.12.x
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.