Cache ResultCaps

[odersky]

This fixes problems encountered for #15923 where we got infinite recursions
caused by infinite streams of new ResultCap instances that were added to capture sets.
The analysis of the problem showed that there is a cycle of dependencies involving
both forwards and backwards propagations of ResultCap instances between
BiMapped capture sets linked by SubstBindingMaps. Each propagation would
create a new derived ResultCap instance.

We could try to solve the problem by having SubstBindingMaps remember their mappings
and have their inverses work backwards. But that could still produce an infinite stream
if there was a cycle of SubstBindingMaps of length > 2. So we fix the problem at the
root by allowing only one derived ResultCap instance per original ResultCap / binder pair.

Fixes #15923

Based on #23184. Only last commit is new.

[Linyxus]

LGTM!

however, the soundness issue I mentioned does compile on the branch too:

import language.experimental.captureChecking
trait Cap
case class Id[X](x: X)
def mkId[X](x: X): Id[X] = Id(x)
def withCap[X](op: (lcap: Cap^) => () => X): X = ???
def bar() =
  val f = (lcap: Cap^) => () => mkId(lcap)
  val leak = withCap(f)
  val leaked = leak.x

Seems to be related to capture sets for inferred types.

[Linyxus]

The tree after cc:

    def bar(): Unit =
      {
        val f: (x$0: Cap^) ->{} () ->{} Id[box Cap^{x$0}]^{} = (lcap: Cap^) => () => mkId[box Cap^{lcap}](lcap)
        val leak: Id[box Cap^{}]^{} = withCap[box Id[box Cap^?]^?](f)
        val leaked: Cap^{} = leak.x
        ()
      }

note that f has the type (x$0: Cap^) -> () -> Id[box Cap^{x$0}] but when passing f to withCap the capture set in the inferred type argument seems to be empty (box Id[box Cap^?]).

[odersky]

The tree after cc:

    def bar(): Unit =
      {
        val f: (x$0: Cap^) ->{} () ->{} Id[box Cap^{x$0}]^{} = (lcap: Cap^) => () => mkId[box Cap^{lcap}](lcap)
        val leak: Id[box Cap^{}]^{} = withCap[box Id[box Cap^?]^?](f)
        val leaked: Cap^{} = leak.x
        ()
      }

note that f has the type (x$0: Cap^) -> () -> Id[box Cap^{x$0}] but when passing f to withCap the capture set in the inferred type argument seems to be empty (box Id[box Cap^?]).

Two observations:

• An error is issued if X is declared covariant in Id.
• If we drop the inner () => in withCap and f we get the same behavior - error for +X, no error for X.

[Linyxus]

look great to me !