Detect and remove __anon_exprs in repl

You might have noticed that when entering in top-level expressions you
end up getting duplicate __anon_expr.1, __anon_expr.2s etc. Since we
don't want these anonymous functions to stick around, in this commit
we're detecting them and then removing them once we're done.

We also only want to run the JIT whenever the user has entered a
top-level expression, so we've also sketched that out.