Add a close_chain endpoint for contracts.

[afck]

Applications should be given the opportunity to clean up, e.g. unsubscribe from everything, when a chain is closed.

[afck]

I'm not convinced anymore that a close_chain endpoint on the contract is a good idea: What if running all the close_chain calls on all installed applications takes too much gas for a single block?

Maybe it should rather be an uninstall endpoint that allows deregistering apps from chains. CloseChain would then fail if there are any applications left—and have a flag to override this and close the chain anyway.

Or each application can optionally provide custom cleanup functions, and have a flag recognized by the system whether it's okay to deregister it. Applications that do require cleanup can set this flag to false until cleanup has been performed.

[afck]

Another use case is for e.g. the fungible or non-fungible applications to move all assets elsewhere before closing a chain.

[afck]

Done in https://github.com/linera-io/linera-protocol/pull/1746.