[coro.generator] Rename the generator's template parameter V to Val

[hewillk]

Fixes #6451.

[jensmaurer]

Names of template parameters rarely convey lots of meaning, it seems to me. So, this change feels rather undermotivated to me.

@jwakely ?

[jwakely]

While I agree that template parameter names aren't always meaningful, for generator we have Ref and V not just T and U or T1 and T2, so there's already some attempt at making them meaningful.

And I think there's also scope for confusion due to class V = void and is_void_v<V> making it seem like V is for void.

I think this change does give some positive value (no pun intended).

[jwakely]

And I think there's also scope for confusion due to class V = void and is_void_v<V> making it seem like V is for void.

Ha, just realised I've repeated the argument given in #6451, so I guess that means I agree with it.

[jensmaurer]

Let's do that, then.

[tkoeppe]

@hewillk In the future, can you please write commit messages that describe the change?