Load contract state lazily in cargo-concordium

[abizjak]

Task description

Currently cargo-concordium loads the entire contract state into memory before executing. This is probably good enough for most practical uses, but it is nevertheless wasteful.

Contract state supports lazy loading of state. Only the state root needs to be loaded upfront, the rest on demand.

This task is to revise how cargo-concordium stores and loads contract state. Essentially the change needs to be made so that state is not "serialized" and "deserialized", but instead "store_update" and "load" are used, with a backing store.