SI-6448 Collecting the spoils of PartialFun#runWith

Avoids calling both `isDefinedAt` and `apply`. This pathological
case that would benefit the most looks like:

  xs collect {
    case x if {expensive(); true} => x
  }

The typical change looks like:

-    for (x <- this) if (pf.isDefinedAt(x)) b += pf(x)
+    foreach(pf.runWith(b += _))

Incorporates feedback provided by Pavel Pavlov:

  https://github.com/retronym/scala/commit/ef5430

A few more opportunities for optimization are noted in the
`Pending` section of the enclosed test. `Iterator.collect`
would be nice, but a solution eludes me.

Calling the guard less frequently does change the behaviour
of these functions in an obervable way, but not contravene
the documented semantics. That said, there is an alternative
opinion on the comment of the ticket:

  https://issues.scala-lang.org/browse/SI-6448

1 files changed, 32 insertions, 0 deletions

diff --git a/test/files/run/t6448.check b/test/files/run/t6448.check
new file mode 100644
index 0000000000..9401568319
--- /dev/null
+++ b/test/files/run/t6448.check
@@ -0,0 +1,32 @@
+
+=List.collect=
+f(1)
+f(2)
+List(1)
+
+=List.collectFirst=
+f(1)
+Some(1)
+
+=Option.collect=
+f(1)
+Some(1)
+
+=Option.collect=
+f(2)
+None
+
+=Stream.collect=
+f(1)
+f(2)
+List(1)
+
+=Stream.collectFirst=
+f(1)
+Some(1)
+
+=ParVector.collect=
+(ParVector(1),2)
+
+=ParArray.collect=
+(ParArray(1),2)