Add a proper write barrier for MVars

Add a proper write barrier for MVars
Previously MVars were always on the mutable list of the old
generation, which meant every MVar was visited during every minor GC.
With lots of MVars hanging around, this gets expensive.  We addressed
this problem for MUT_VARs (aka IORefs) a while ago, the solution is to
use a traditional GC write-barrier when the object is modified.  This
patch does the same thing for MVars.

TVars are still done the old way, they could probably benefit from the
same treatment too.