x*? is x.type @reach @maybe. This was not recognized before.

Previously, we violated that assumption is we too the deep capture set
of a capture reference wiht singleton type.

Count in dcs exactly those locations where a cap gets replaced by a reach capability.

Simplify code that handles applications, avoiding adding pieces of mutable state.

The correct point to address charging reach capabilities is in markFree itself:
When a reach capability goes out of scope, and that capability is not a parameter,
we need to continue with the underlying capture set.

Drop the scheme where we only charge the last arrow of a curried lambda with
elements used in the body. On the one hand, this is unsound without compensation
measures (like, restricting to reach capabilities, or taking all capture sets
of a named curried function as the underlying reference). On the other hand, this
should be generalized to all closures and anonymous functions forming the right
hand sides of methods.

Use sets of nested methods are anyway charged on call.

This gives better error messages. Previously we thought this would
make reach capabilities unsound, but I don't see an issue with the latest
design.

With our current @use scheme, this is unsound. We leave the possibility to re-enable
as a Config option which is disabled by default and comes with a warning that enabling
it would be unsound.

Check that type parameters of methods and parent traits don't get instantiated
with types containing a `cap` anywhere in covariant or invariant position.

Also: add test that reach capabilities are contained inside boxes

…esult type

We can use the dcs only if there are no type variables.

The additional purity in the asInstanceOf target is not needed

Retain only caps.unsafe.unsafeAssumePure

Don't show an `(ex$n: Exists) ->` if the bound variable
does not appear in the result. The full type will be shown under -Ycc-debug.

Also, avoid spurious ineffective mappings in widenReach.

A by-name Closure node, which is produced by phase ElimByName gets a target type to indicate
it's a contextual zero parameter closure. But for the purposes of rechecking and capture checking,
it needs to be treated like a function. In particular the type of the closure needs to be derived
from the result type of the anonymous function.

Fixes scala#21920