Refine rules for capture parameters and members

[noti0na1]

This PR refines rules for capture variables (parameters and members).

Fix #21999, #22005

Add requirements for capture variables

When a capture set is encoded as a type, the type must refer to CapSet and bounded by >: CapSet <: CapSet^.

An unbounded capture parameter would be C >: CapSet <: CapSet^, which can be desugared from C^.

class A:
  type C^

def f[C^](io: IO^{C^}) = ???

// becomes

class A:
  type C >: CapSet <: CapSet^

def f[C >: CapSet <: CapSet^](io: IO^{C^}) = ???

Then, constaints between capture variables become possible:

def test[X^, Y^, Z >: X <: Y](x: C^{X^}, y: C^{Y^}, z: C^{Z^}) = ???
// Z is still bounded by >: CapSet <: CapSet^

Update definitions in the library caps.scala, such that a type following the rule can be used inside a capture set.

// Rule out C^{(Nothing)^} during typer
def capsOf[CS >: CapSet <: CapSet @retainsCap]: Any = ???

sealed trait Contains[+C >: CapSet <: CapSet @retainsCap, R <: Singleton]

Add cases to handle CapSet in subsumes

*   X = CapSet^cx, exists rx in cx, rx subsumes y ==>  X subsumes y
*   Y = CapSet^cy, forall ry in cy, x subsumes ry ==>  x subsumes Y
*   X: CapSet^c1...CapSet^c2, (CapSet^c1) subsumes y  ==> X subsumes y
*   Y: CapSet^c1...CapSet^c2, x subsumes (CapSet^c2) ==> x subsumes Y
*   Contains[X, y]  ==>  X subsumes y

[noti0na1]

TODO: fix override check for type members during CC

[bracevac]

This should not be necessary. I'll look into normalization and subtyping of captures.