Fixing a regression from #250; adding some sync var-related tests+notes
Merge pull request #289 from vasslitvinov/syncvar-followups-to-coercion-wrapper-elimination

Fixed a regression from #250; added some sync var-related tests+notes.

* Fixed this regression from #250 aka coercion wrapper elimination:

[Error matching .bad file for users/vass/type-tests.todo]

* This .bad mismatch exposed a change of behavior of that test.
It used to be a compiler error; now it goes through the compiler
and executes incorrectly.

* To document this, I created a separate future for that,
with a few variants while I was on that.

* All of this applies equally to single variables;
I did not take the time to create single-var variants of these tests.

The only exception is ref-sync-6-*, which are the 'single'
counterparts of ref-sync-5-*.

* I noted that sync/single-related tests are in two places
in our test tree, by creating two .SEE_ALSO.txt files:

  test/types/sync
  test/parallel/sync

  test/types/single
  test/parallel/single

* Added a note to jbreitbart.future about allowing syncs/singles
of syncs/singles.