Fix apply rule

[odersky]

Don't perform the apply rule optimization under boxing

This was already the case for the select rule optimization in recheckSelection
but was missing for the corresponding rule for applications.

[bracevac]

LGTM

[Linyxus]

That way, this escape hatch becomes unnecessary.

[odersky]

But I believe it's still good to have the escape hatch, in general.

[odersky]

This caused one regression in the standard library, here:

  private def eagerHeadConcatIterators[A](it: Iterator[collection.Iterable[A]^]^): LazyListIterable[A]^{it} =
    if !it.hasNext then Empty
    else
      eagerHeadPrependIterator
          (it.next().iterator)
          (eagerHeadConcatIterators(it))

Previously the access to it.next() was considered to have type it by applying the apply rule
incorrectly. It should be it*. This means we now have it* instead of it in the result type
and we also have an illegal use of it* leaking outside eagerHeadConcatIterators. The second
problem was fixed by adding an unsafe escape hatch unsafeDiscardUses that suppressed use recording.
This leads to:

  private def eagerHeadConcatIterators[A](it: Iterator[collection.Iterable[A]^]^): LazyListIterable[A]^{it*} =
    if !it.hasNext then Empty
    else
      eagerHeadPrependIterator
          (caps.unsafe.unsafeDiscardUses(it.next()).iterator)
          (eagerHeadConcatIterators(it))

This also did not compile since it claimed that the it @reachCapability was not a legal element of a capture set.
The root cause was that we forced some definitions already in parser, which can lead to confusion when compiling the
standard library itself. We now refrain from doing that and build the references to these annotations as untyped
trees all the way down.

[natsukagami]

we also have an illegal use of it* leaking outside eagerHeadConcatIterators

Is this where we should annotate @use?

[odersky]

We don't have @use anymore.