Tag serialised client values

[dhil]

In patch #1015 we added tags to server side values in order to make it easy to deserialise them on the client side. Currently, client side values are passed as serialised raw runtime objects to the server side during remote calls. As a consequence, the deserialisation on the server side uses a fair amount of guesswork and backtracking to deserialise a client side value.

To make server-side deserialisation easy, we should tag client side values too. There is an argument to be made, that we may want to keep objects tagged, currently, the client side performs tag erasure during deserialisation -- the cost of keeping objects tagged ought to be tiny on both the client and server side.

Related issues #219 and #1050.