Issue-2598: Fix memory leaks and minimisations.

[diesalbla]

From #2598, we add a leak test for pure streams, one with flatMap and uncons, which does not involve parallelism.

We then make the changes in pull interpreter that would fix that specific leak test case.

[mpilquist]

Memory tests pass but looks like we're getting a stack overflow exception on map:

fs2/core/shared/src/test/scala/fs2/StreamSuite.scala

Lines 455 to 461 in 5d4436a

	test("regression #2353 - stack safety of map") {
	@tailrec
	def loop(str: Stream[Pure, Int], i: Int): Stream[Pure, Int] =
	if (i == 0) str else loop(str.map((x: Int) => x + 1), i - 1)
	loop(Stream.emit(1), 10000).compile.toList //
	}

  + map - regression #1335 - stack safety of map 0.004s
java.lang.StackOverflowError
  | => cat fs2.Pull$.fs2$Pull$$viewL$1(Pull.scala:839)
	at fs2.Pull$.fs2$Pull$$go$1(Pull.scala:1161)
	at fs2.Pull$.fs2$Pull$$go$1(Pull.scala:1190)
	at fs2.Pull$.fs2$Pull$$go$1(Pull.scala:1190)
	at fs2.Pull$.fs2$Pull$$go$1(Pull.scala:1190)
	at fs2.Pull$.fs2$Pull$$go$1(Pull.scala:1190)
	at fs2.Pull$.fs2$Pull$$go$1(Pull.scala:1190)
	at fs2.Pull$.fs2$Pull$$go$1(Pull.scala:1190)
	at fs2.Pull$.fs2$Pull$$go$1(Pull.scala:1190)
	at fs2.Pull$.fs2$Pull$$go$1(Pull.scala:1190)
	at fs2.Pull$.fs2$Pull$$go$1(Pull.scala:1190)
	at fs2.Pull$.fs2$Pull$$go$1(Pull.scala:1190)

[diesalbla]

@mpilquist That stack overflow is similar to the one that appears with FlatMap, because the call to go has a left-recursive call to go. Adding a F.unit >> in front of it seems to delay that recursive call.