Implement quote auto-lifting under an import

The idea is to have an implicit conversion that converts a `T` to an `Expr[T]` using the `toExpr` conversion. This would allow us to write

```scala
def plus(n: Int, m: Expr[Int]): Expr[Int] = 
  if (n == 0) m else '{ $n + $m }
```
instead of 
```scala
def plus(n: Int, m: Expr[Int]): Expr[Int] = 
  if (n == 0) m else '{ ${n.toExpr} + $m }
```