add capture-checked scala.concurrent standard library classes

[olhotak]

Adds a capture-checked version of the scala.concurrent standard library classes to scala2-library-cc.

Marked as draft for now because it fails separation checking.

[olhotak]

Here are the separation checking errors:

[error] -- Error: /home/olhotak/git/dotty/scala2-library-cc/src/scala/concurrent/ExecutionContext.scala:291:77 
[error] 291 |  def fromExecutor(e: Executor): ExecutionContextExecutor^ = fromExecutor(e, defaultReporter)
[error]     |                                                                             ^^^^^^^^^^^^^^^
[error]     |Separation failure: Illegal access to cap which is hidden by the previous definition
[error]     |of method fromExecutorService with result type scala.concurrent.ExecutionContextExecutorService^.
[error]     |This type hides capabilities  {cap}
[error] -- Error: /home/olhotak/git/dotty/scala2-library-cc/src/scala/concurrent/impl/Promise.scala:74:39 
[error] 74 |      compressed(current = c, target = c, owner = owner)
[error]    |                                       ^
[error]    |Separation failure: argument of type  (c : scala.concurrent.impl.Promise.DefaultPromise[box T^?]^{Link.this.to})
[error]    |to method compressed: (current: scala.concurrent.impl.Promise.DefaultPromise[T]^{cap.rd, Link.this},
[error]    |  target: scala.concurrent.impl.Promise.DefaultPromise[T]^, owner:
[error]    |  scala.concurrent.impl.Promise.DefaultPromise[T]^):
[error]    |  scala.concurrent.impl.Promise.DefaultPromise[T]^{Link.this, current, target,
[error]    |    owner}
[error]    |corresponds to capture-polymorphic formal parameter target of type  scala.concurrent.impl.Promise.DefaultPromise[T]^
[error]    |and hides capabilities  {c}.
[error]    |Some of these overlap with the captures of the function prefix with type  (Link.this : scala.concurrent.impl.Promise.Link[T]^{Link.this.to}).
[error]    |
[error]    |  Hidden set of current argument        : {c}
[error]    |  Hidden footprint of current argument  : {c, Link.this.to}
[error]    |  Capture set of function prefix        : {Link.this}
[error]    |  Footprint set of function prefix      : {Link.this, Link.this.to}
[error]    |  The two sets overlap at               : {Link.this.to}
[error] -- Error: /home/olhotak/git/dotty/scala2-library-cc/src/scala/concurrent/impl/Promise.scala:167:45 
[error] 167 |              zipped.tryComplete(try Success(f(left.get, right.get)) catch { case e if NonFatal(e) => Failure(e) })
[error]     |                                             ^
[error]     |Separation failure: Illegal access to {f} which is hidden by the previous definition
[error]     |of value thisF with type scala.util.Try[T] => Unit.
[error]     |This type hides capabilities  {buffer, zipped, f}
[error] -- Error: /home/olhotak/git/dotty/scala2-library-cc/src/scala/concurrent/impl/Promise.scala:363:28 
[error] 363 |          } else linkRootOf(p, l)
[error]     |                            ^
[error]     |Separation failure: argument of type  (p :
[error]     |  scala.concurrent.impl.Promise.DefaultPromise[box T^?]^{l, DefaultPromise.this}
[error]     |  )
[error]     |to method linkRootOf: (target: scala.concurrent.impl.Promise.DefaultPromise[T]^, link:
[error]     |  scala.concurrent.impl.Promise.Link[T]^): Unit
[error]     |corresponds to capture-polymorphic formal parameter target of type  scala.concurrent.impl.Promise.DefaultPromise[T]^
[error]     |and hides capabilities  {p}.
[error]     |Some of these overlap with the captures of the second argument with type  scala.concurrent.impl.Promise.Link[box T^?]{
[error]     |  val to: scala.concurrent.impl.Promise.DefaultPromise[box T^?]^{l*}}^{l}.
[error]     |
[error]     |  Hidden set of current argument        : {p}
[error]     |  Hidden footprint of current argument  : {p, l, DefaultPromise.this, link, target}
[error]     |  Capture set of second argument        : {l*}
[error]     |  Footprint set of second argument      : {l*, link, target}
[error]     |  The two sets overlap at               : {link, target}
[error] four errors found

[olhotak]

Compiles if we change useSepChecks to false in CaptureOps.scala.

[natsukagami]

We should look into this more, I tried simplifying this signature but it seems separation checker is not trying to shorten paths