StaticPointers: Refer to PackageKey in comments and field name of StaticPtrInfo.
Trac #9878: Make the static form illegal in interpreted mode.

The entries of the static pointers table are expected to exist
as object code. Thus we have ghci complain with an intelligible
error message if the static form is used in interpreted mode.
Trac #9878: Use a mutex to protect the spt.
StaticPointers: Make monadic unsafeLookupStaticPtr and staticPtrKeys.
StaticPointers: Remove SPT entries when unloading a module.