Add feature for evaluating and reducing synthetic quotations with unbound variables

```
Now that we have our own Eval implementation, we can easily add Expr extension 
members to Reduce, ReduceFully, and the new Eval so that we can pass in an 
variable environment to handle synthetic quotations, e.g.

    Expr.Var(new Var("x", typeof<int>)).Eval([("x", 1 |> box |> ref)])

```

Original issue reported on code.google.com by `stephen....@gmail.com` on 4 Jul 2011 at 4:40
