Add sendable to core library

[thomasvl]

No description provided.

[thomasvl]

@zwaldowski -

#1213 added the TODO in more places, but a few of them already have the extension to add @unchecked Sendable, what was the intent in the other cases?

[zwaldowski]

Yeah, I suppose the TODO on UnknownStorage is a duplicate; that's easily resolved.

what was the intent in the other cases?

The others are all related to ExtensionFieldValueSet. ExtensionFieldValueSet is a requirement of ExtensibleMessage, so moving towards a world of Sendable instead of @unchecked Sendable, it should be Sendable. That family of types can't participate in the _ProtoSendable trick because ProtobufBytes has an associated type of Data, which can't satisfy an associated type requirement for FieldType.BaseType: _ProtoSendable for the same reasons involving Data elsewhere.

I don't think there's anything to practically be done about those TODOs until Data is Sendable or it's replaced with a non-Foundation ContiguousBytes type that is Sendable.

[thomasvl]

Thanks, updated to remove the comment on UnknownStorage, but leaving the rest.

[thomasvl]

@zwaldowski & @Lukasa - if you can also look things over, it would be appreciated.

[Lukasa]

Seems reasonable to me.