Context for Map

[FintanH]

It seems strange to call a function like len or is_empty just to get the ReadCtx for a Map. I'd like to suggest adding a function read_ctx that returns ReadCtx<(), A> for this use case.

Also updating the example showing the use of it.

[davidrusu]

Yep, makes sense. We have this implemented for the Orswot already: https://github.com/rust-crdt/rust-crdt/blob/master/src/orswot.rs#L274-L280

Would be good to bring this to the Map as well