SI-8990 Allow GC during lazy evaluation of Stream#withFilter

  - The fact that StreamWithFilter was an inner class prevented
    collection of the head during processing after #withFilter,
    due to the reference to the outer class instance held in self.
  - By implementing StreamWithFilter outside of the Stream class,
    we gain control of the reference to the original Stream.
  - We clarify explicit "filter after first use" semantics for the
    reference to the original stream, which allows it to be GC'd
    during the processing of foreach, map, and flatMap.
  - Code is more DRY by implementing in terms of Stream#filter,
    which is already correctly lazy, and then Stream's #map,
    #flatMap, and #foreach, which already correctly allow GC.
  - Unfortunately, the type returned by Stream#withFilter *must*
    change, as it had previously inherited from the inner class
    TraversableLike#WithFilter, which is where the problematic
    reference to the outer class instance was held. Therefore,
    this change is targetted to 2.12.x. There doesn't appear to
    be any way to fix this without changing the type, sadly.
  - Special thanks to @paulp who suggested the likely cause of the
    issue at nescala 2015, and got me setup to build and run tests.
    Also thanks to @Ichoran and @retronym, who suggested that
    filter-after-first-use would preserve the reusable nature
    of the return value from #withFilter, rather than a single-shot
    null-after-first-use solution.

Review by @Ichoran and @retronym
Fixes #8990

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