Represent read-only with a classifier

[odersky]

x.rd is now a shorthand for x.only[caps.Read].

[Linyxus]

It is surprising that this PR also fixes #24334.

[odersky]

@natsukagami ping for review

[natsukagami]

I was slightly concerned with readonly not being a .only[Read] if we wanted to drop the cannot-capture-different-classifier restriction, but since we will take another route I think it's fine.

[natsukagami]

Is it valid here to strip the existing restricted? e.g. if the classifiers was extended with a sub-classifier of Read

[odersky]

We don't support restrictions with more than one classifierr, so the only issue here is that we might want to keep the existing sub-classifier of Read (if such things exist). But I think that should be handled by the caller.

[Linyxus]

But we said Mutable should be a special "classifier" that allows its instance to capture anything. So we might well have a matrix m: Matrix^{fresh1, async} that captures a control capability Async (here the fresh1 is the Mutable fresh for this matrix). Then, getting readonly on m.only[Control].rd should, by principle, be empty. But with the current logic it will be m.rd or equivalently fresh1.rd.

[Linyxus]

After a second thought I think it should be fine. This expressiveness issue could be solved by first widening m.only[Control] to just async and then take readonly on it.

[Linyxus]

After a second thought I think it should be fine. This expressiveness issue could be solved by first widening m.only[Control] to just async and then take readonly on it, which will be empty.