Squashed 'native/sylvan/' changes from 338ae7e..ac97e16

ac97e16 Replace internal reference management by new data structure
f61f8de Replace LLCI cache by new cache data structure.
d8ddb22 Add extra assertion to sylvan_test_isset to check if an item is properly marked
61f0abc Bugfix: properly round up when marking external references
ad0d9f8 Minor optimization in llmsset
e15abca llmsset: search for a free slot for new BDD nodes now starts at random location
b77bf1f Minor modification of commented debug code that reports statistics on garbage collection.
f080dd7 Minor improvements to README.md
e1ea870 Major rewrite of mc example
8a1a1d0 Added URLs to Java and Haskell bindings to README.md

git-subtree-dir: native/sylvan
git-subtree-split: ac97e1635c04c4b33b764b688471a02902bc4f18
Update Sylvan to latest version