Squashed 'native/sylvan/' changes from ac97e16..90bc6b1

90bc6b1 Bugfix: garbage collection did not properly mark BDDs that were the result of stolen and completed tasks. Now, every spawned Sylvan task is stored in a thread-local array. During garbage collection, the results of completed tasks are now also marked as in use.

git-subtree-dir: native/sylvan
git-subtree-split: 90bc6b1ca92e4b2c50f11ddfb85912ab89ea4469
Update Sylvan to latest version