Revert Stream.Cons to the 2.12 encoding

[szeiger]

• The lazy val required an extra bitmap field which increases the
  instance size from 24 to 32 bytes.
• Capturing tl in the initialization code prevented it from being
  garbage collected as soon as the tail has been computed and the
  initializer is no longer needed.

Fixes scala/bug#11696, at least in theory.
I was unable to reproduce the failure with the code from the original
bug report.

LazyList is not affected. It already releases the initializer after
running it, and the instance size (on x64) is 24 bytes with or without
the bitmap field for the lazy val.

[szeiger]

I'm not sure what to make of the Dotty error:

[error] -- [E099] Syntax Error: /home/travis/build/scala/scala/src/library/scala/collection/immutable/Stream.scala:376:43 
[error] 376 |    @volatile private[this] var tlGen = tl _
[error]     |                                        ^^^^
[error]     |Only function types can be followed by _ but the current expression has type Stream[A]

This is the same as 2.12 and it's also found in the Dotty repo: https://github.com/lampepfl/dotty/blob/master/tests/scala2-library/src/library/scala/collection/immutable/Stream.scala#L1163

[allanrenucci]

It is a warning under -language:Scala2. You would need to write:

@volatile private[this] var tlGen = () => tl

[NthPortal]

LGTM