Implement toString for keys

toString added for REPL testing:

```
scala> Zero / Zero / Zero / name
res0: sbt.SlashSyntax.ScopeAndKey[sbt.SettingKey[String]] = Zero / Zero / Zero / name
```

main-settings/src/main/scala/sbt/Scope.scala

@@ -24,6 +24,11 @@ final case class Scope(project: ScopeAxis[Reference],
   def in(project: Reference): Scope = copy(project = Select(project))
   def in(config: ConfigKey): Scope = copy(config = Select(config))
   def in(task: AttributeKey[_]): Scope = copy(task = Select(task))
+
+  override def toString: String = {
+    if (extra == This) s"$project / $config / $task"
+    else s"Scope($project, $config, $task, $extra)"
+  }
 }
 object Scope {
   val ThisScope: Scope = Scope(This, This, This, This)

main-settings/src/main/scala/sbt/Structure.scala

@@ -34,6 +34,8 @@ sealed abstract class SettingKey[T]
 
   val key: AttributeKey[T]
 
+  override def toString: String = s"SettingKey($scope / $key)"
+
   final def toTask: Initialize[Task[T]] = this apply inlineTask
 
   final def scopedKey: ScopedKey[T] = ScopedKey(scope, key)
@@ -105,6 +107,8 @@ sealed abstract class TaskKey[T]
 
   val key: AttributeKey[Task[T]]
 
+  override def toString: String = s"TaskKey($scope / $key)"
+
   def toTask: Initialize[Task[T]] = this
 
   def scopedKey: ScopedKey[Task[T]] = ScopedKey(scope, key)
@@ -172,6 +176,8 @@ sealed trait InputKey[T]
 
   val key: AttributeKey[InputTask[T]]
 
+  override def toString: String = s"InputKey($scope / $key)"
+
   def scopedKey: ScopedKey[InputTask[T]] = ScopedKey(scope, key)
 
   def in(scope: Scope): InputKey[T] =

main/src/main/scala/sbt/SlashSyntax.scala

@@ -97,6 +97,10 @@ object SlashSyntax {
   final class ScopeAndKey[K <: Key[K]](scope: Scope, key: K) {
     private[sbt] def materialize: K = key in scope
     private[sbt] def rescope: TerminalScope = new TerminalScope(scope in key.key)
+
+    override def toString: String = {
+      s"$scope / ${key.key}"
+    }
   }
 
 }