A big improvement to the way command-line expressions are typechecked.
Now we don't wrap in "print" and hope for the best (the wrong "print"
might be in scope).  Instead we work on the renamed epxression and
do the Right Thing by using the correct "print".

Also do generalisation, so that we get the right type back from
the :t command.

WARNING: it's possible that these files overlap with my fortcoming
Big Commit of typechecker stuff, so you may need to hang on for
a few mins.