Implement seal_set_code_hash and add example contract

[cmichi]

I've marked this issues as a good first one since the task is something that can be done mostly analog to already existing code.

ink! interfaces with Substrate's contracts pallet via an API. The API functions are named with the prefix seal_ ‒ so e.g. seal_call. From time to time new API functions are added. One recently added one is seal_set_code_hash, you can find it here: https://github.com/paritytech/substrate/blob/master/frame/contracts/src/wasm/runtime.rs#L1976-L2011. It was added in paritytech/substrate#10567.

While the contracts pallet side has been implemented we are missing a possibility to invoke this function from ink!.

ToDo

• Add a new function ink_env::set_code_hash.
  Take a look at the file env/src/api.rs, it should be possible to take one of the functions there (e.g. is_contract) and implement this new function analog.
• You will have to add seal_set_code_hash in this file: https://github.com/paritytech/ink/blob/master/crates/env/src/engine/on_chain/ext.rs#L341. Add it as an __unstable__ function there.
• You can ignore off-chain testing for now and add an unimplemented!("off-chain environment does not yet support …"); there.
• Add an example which illustrates how this new function can be used to write an upgradeable smart contract.