First shot at a STG interpreter for GHCI.  Translates Stg syntax into a
form convenient for interpretation, and can then run that.  Most of the
translation stuff is there and works.  The interpreter framework is there
and partly filled in, and seems to work.  There are still quite a lot of
cases, etc, to fill in, but this should be straightforward given that
the framework exists.  This interpreter cannot handle (yet?) unboxed
tuples, but can deal with more or less everything else, including standard
unboxed Int, Double, etc, code.