Add restricted capabilities x.only[C]

[odersky]

Implementation along the lines of #23463

• Syntax and parsing rules for only-capabilities.
• A Classifier base trait.
• Enforce restriction that a class cannot inherit two unrelated classified capability classes.
• A new capability class for only-capabilities.
• Implement captureSetOfInfo for only capabilities.
• The empty capability is x.only[Nothing]. It should have empty captureSetOfInfo.
• Well-formedness rules: only must refer to a classified capability class.
• Normalization rules:
  - it's *, then .only, then .rd,
  - multiple .only normalize to the smallest one if the classes are related,
  - multiple .only normalize to the empty capability if the classes are not related.
• define transitive capture set and cache it in a capability. The tcs does not exist if a capture set in the hierarchy is still an unsolved capture set variable.
• Add a classifier field to FreshCap and ResultCap.
• Modify capToFresh and toResultInResults so that the classifier field is correctly set.
• Using tcs, define when a capability is classified by a classifier class.
• A FreshCap with a classifier C can subsume only capabilities that are classified as C.
• Implement subsumption rules:
  - c.as[C] <: d if c <: d or c.as[D] <: empty
  - c.as[C] <: d.as[D] if c <: d and C derives from D
  - c <: d.as[D] if c <: d and c is classified as D
  - c.as[D] <: empty if tcs(c) consists of capabilities that all derive from classifier classes unrelated to D.

[prolativ]

Suggested change

	* References that extend cpas.Sharable are excluded from checking.
	* References that extend caps.Sharable are excluded from checking.

[odersky]

👍