Add String to Liftable expressions and abstract over all primitives

Author: nicolasstucki

library/src/scala/quoted/Expr.scala

@@ -2,7 +2,7 @@ package scala.quoted
 
 import scala.runtime.quoted.Runner
 
-abstract class Expr[T] extends Quoted {
+trait Expr[T] extends Quoted {
   final def unary_~ : T = throw new Error("~ should have been compiled away")
   final def run(implicit runner: Runner[T]): T = runner.run(this)
   final def show(implicit runner: Runner[T]): String = runner.show(this)