A stream wrapped into a Body previously needed to implement `Sync` so
that the Body type implements this autotrait as well (which is needed
due to limitations in async/await). Since a stream only offers one
method that is called with an exclusive reference, this type is
statically proven to be Sync already. In theory it should be fine to add
an `unsafe impl Sync`, but this commit instead adds a SyncWrapper to
enlist the compiler’s help in proving that this is (and remains) correct.
This makes it easier to construct response bodies for client code.
