Fix issue 87: gc should scan callback
This causes the extraRoots to get scanned during gc.
Change it so we don't run GC every time we pause Haskell.
This is a bit of a performance killer in my WebGL Freecell, for example,
because it pauses very frequently (for every animation frame) and there
are quite a lot of live objects to scan.
This change makes it check the gc interval when we pause, like we do in
the other places.
Merge pull request #1 from the-real-blackh/master

Fix issue 87: gc should scan callback