SharedString and SharedVector should be Send

SharedString used to be Send, but this regress when we moved the implementation
over to SharedVector.
This regression was cought trying to compile CargoUI with master

sixtyfps_runtime/corelib/sharedvector.rs

@@ -86,6 +86,9 @@ pub struct SharedVector<T> {
     inner: NonNull<SharedVectorInner<T>>,
 }
 
+// Safety: We use atomic reference counting, and if T is Send, we can send the vector to another thread
+unsafe impl<T: Send> Send for SharedVector<T> {}
+
 impl<T> Drop for SharedVector<T> {
     fn drop(&mut self) {
         unsafe {