Make the dynamic linker thread-safe.

The current implementation is rather pessimistic.  The persistent
linker state is now an MVar and all exported Linker functions are
wrapped in modifyMVar calls.  This is serves as a big lock around all
linker functions.

There might be a chance for more concurrency in a few places. E.g.,
extending the closure environment and loading packages might be
independent in some cases.  But for now it's better to be on the safe
side.