@_eagerMove in swift-collections

Is it safe to add @_eagerMove to the COW types like Deque in swift-collections? It could be done conditionally via #if compiler(>=5.10).

I'll need to get up to speed on its effects, but I think so! Patches are welcome. :wink:

(In particular, if it weakens the lifetime guarantees on borrowed self methods, then we may need to add some new _fixLifetime/withExtendedLifetime calls. Then again, the UnsafeHandle pattern the package uses may mostly take care of that.)