Substrate: contracts: implement `ext_runtime_get_storage`

[ascjones]

In order to allow smart contracts to query runtime storage, a new ext_ function is required:

fn ext_runtime_get_storage(key_ptr: u32, key_len: u32) -> u32

I've implemented a quick prototype: https://github.com/paritytech/substrate/compare/aj-contracts-get-storage.

However this is hard coded to hash the key twox_128(key)

Assuming we don't expect the contract itself to perform the key hashing, the remaining problem to solve is the API for correct hashing of keys for different storage types:

• Value twox_128(key)
• Map blake2_256(prefix + key)
• Map2 twox_128(prefix + key)
• LinkedMap blake2_256(prefix + key)
• LinkedMap2 twox_128(prefix + key)
• DoubleMap blake2_256(prefix + key) + blake2_256(key2)
• DoubleMap2 twox_128(prefix + key) + twox_128(key2)

Rel: https://github.com/paritytech/ink/pull/204

[xlc]

Related previous discussion: #2214

[xlc]

Access the storage directly will make it very hard / impossible to upgrade runtime module storage format because it means all the contracts have to upgrade as well, which is just not going to happen in a generic smart contract chain.