Fix T: Sync bound of impl Send for ViewStorage

[197g]

Closes: #1001

Note that this is a breaking change. The precondition differs between ViewStorage and ViewStorageMut just like they differ between &mut T and & T. While both allow reborrowing a &T from a shared reference to them, i.e. they have the same requirements for their Sync implementations, the formalism is different for the Send trait:

• a unique reference makes the value itself available and its creation and lifetime requires synchronization itself, hence Send where T: Send.
• a shared reference does not imply any synchronization point in its creation or during its lifetime so we must only make it available to parallel use (sendable to other threads) if the underlying type handles that parallelism, hence Send where T: Sync.

See: https://doc.rust-lang.org/stable/std/primitive.reference.html#trait-implementations-1

[Ralith]

Thanks! I won't merge this personally so it doesn't catch @sebcrozet by surprise, but we should definitely take this for the next breaking release.

[sebcrozet]

Thank you!