Initial primop support for the metacircular interpreter (GHCI).
Only appears if you compile with -DGHCI; if not, the world is
unchanged.

new primops:
   indexPtrOffClosure#
   indexWordOffClosure#

modified:
   dataToTag#   -- now dereferences indirections before extracting tag

new entry code
   mci_constr_entry          and
   mci_constr[1..8]entry
being the direct and vectored return code fragments for interpreter
created constructors.  Support for static constructors is not yet
done.

New handwritten .hc functions:
   mci_make_constr*
being code to create various flavours of constructors from args
on the stack.  An interface file to describe these will follow in
a later commit.