Carl: Please review slice expression code

[rocky]

27d1dd3ec1ce43da09be2addc34d12f0c915a828

There are these hacky CheckExpr calls:

    if cexpr, errs = CheckExpr(ctx, e.X, env); len(errs) != 0 {
        for _, cerr := range errs {
            fmt.Printf("%v\n", cerr)
        }
        return nil, false, errors.New("Something wrong checking * expression")
    }

[rocky]

Dup with #27