Get External Core (-fext-core) working with readline

Get External Core (-fext-core) working with readline
Had to add support for dynamic C calls and for foreign labels (Addr#
constants). Actually I only did the printing side -- parsing is not
done yet. But at least now you can build the libraries with -fext-core.

I also got the function arrow to print out properly again (it was
printing fully-qualified and z-coded!)

I also added a field for calling convention name to the External
data constructor in ExternalCore.Exp (for static C calls).

I'm not exactly sure where to document all of this, so I haven't done
that, though I did comment the code a bit.