export op class RefUpdateOp from MutableReference, to enable proper subclassing in order to create restricted references

Author: sbazerque

src/data/collections.ts

@@ -1,5 +1,5 @@
 export { MutableSet, MutableSetEvents } from './collections/mutable/MutableSet';
-export { MutableReference } from './collections/mutable/MutableReference';
+export * from './collections/mutable/MutableReference';
 export * from './collections/mutable/MutableArray';
 export * from './collections/causal/CausalSet';
 export { SingleAuthorCausalSet } from './collections/causal/SingleAuthorCausalSet';

src/data/collections/mutable/MutableReference.ts

@@ -211,4 +211,4 @@ class RefUpdateOp<T> extends MutationOp {
 ClassRegistry.register(MutableReference.className, MutableReference);
 ClassRegistry.register(RefUpdateOp.className, RefUpdateOp);
 
-export { MutableReference };
+export { MutableReference, RefUpdateOp };