Increase laziness of #:: for LazyList

[NthPortal]

Change #:: for LazyList so that the tail computation
is wrapped as a state computation for a new LazyList,
thus deferring its execution as long as possible.

Fixes scala/bug#11984

---

Before (2.13.2-):

scala 2.13.2> 1 #:: { println("hey"); LazyList() }
val res4: scala.collection.immutable.LazyList[Int] = LazyList(<not computed>)

scala 2.13.2> res1.head
hey
val res5: Int = 1

After (2.13.3+):

scala 2.13.3> 1 #:: { println("hey"); LazyList() }
val res4: scala.collection.immutable.LazyList[Int] = LazyList(<not computed>)

scala 2.13.3> res1.head
val res5: Int = 1

[NthPortal]

Interestingly, #::: is already sufficiently lazy and doesn't need to be changed. I found it quite difficult to reason about this, however, and was only fully convinced of it after writing a test to prove it.

[SethTisue]

@NthPortal should LazyList.cons have changed as well? see discussion at https://www.reddit.com/r/scala/comments/hgv8k3/lazylist_or_lazylistcons/

scala 2.13.3> 1 #:: { println("hey"); LazyList() }
val res4: scala.collection.immutable.LazyList[Int] = LazyList(<not computed>)

scala 2.13.3> res1.head
val res5: Int = 1

scala 2.13.3> LazyList.cons(1, { println("hey"); LazyList() })
val res6: scala.collection.immutable.LazyList[Int] = LazyList(<not computed>)

scala 2.13.3> res6.head
hey
val res7: Int = 1

[NthPortal]

yeah, probably :(

[NthPortal]

see #9095