OfTypeImplicits: compute refs lazily

Many intermediate `OfTypeImplicits` are created during a call to
`implicitScope`, but they won't all be used so there is no need to
compute `OfTypeImplicits#refs` unless it's actually used.

Author: smarter

src/dotty/tools/dotc/typer/Implicits.scala

@@ -106,7 +106,7 @@ object Implicits {
    */
   class OfTypeImplicits(tp: Type, val companionRefs: TermRefSet)(initctx: Context) extends ImplicitRefs(initctx) {
     assert(initctx.typer != null)
-    val refs: List[TermRef] = {
+    lazy val refs: List[TermRef] = {
       val buf = new mutable.ListBuffer[TermRef]
       for (companion <- companionRefs) buf ++= companion.implicitMembers
       buf.toList