SI-8040 Warn unused parameters

One can `-Ywarn-unused:params` or more narrowly warn only for
unused implicit parameters with `-Ywarn-unused:implicits`.

Params includes constructor parameters.

The settings for privates and locals are not yet distinguished.

```
$ skalac -Ywarn-unused:help
Enable or disable specific `unused' warnings
  imports    Warn if an import selector is not referenced.
  patvars    Warn if a variable bound in a pattern is unused.
  privates   Warn if a private member is unused.
  locals     Warn if a local definition is unused.
  params     Warn if a value parameter is unused.
  implicits  Warn if an implicit parameter is unused.
```

1 files changed, 4 insertions, 0 deletions

diff --git a/test/files/neg/warn-unused-privates.scala b/test/files/neg/warn-unused-privates.scala
index bc2799067e..f0580f02d5 100644
--- a/test/files/neg/warn-unused-privates.scala
+++ b/test/files/neg/warn-unused-privates.scala
@@ -189,3 +189,7 @@ trait Forever {
     } yield 42                           // val emitted only if needed, hence nothing unused
   }
 }
+
+trait Ignorance {
+  private val readResolve = 42      // ignore
+}