Implement quote auto-lifting under an import

[nicolasstucki]

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

def plus(n: Int, m: Expr[Int]): Expr[Int] = 
  if (n == 0) m else '{ $n + $m }

instead of

def plus(n: Int, m: Expr[Int]): Expr[Int] = 
  if (n == 0) m else '{ ${n.toExpr} + $m }

[nicolasstucki]

It would be enabled by importing

import scala.quoted.autolift