Instantiate2 support

[webmaster128]

The goal here is to allow contracts to use instantiate2 to instantiate other contracts (not be instantiated, which works already).

This consists of 3 parts:

1. Allow sending an MsgInstantiateContract2
2. calculate the address such that re don't need to process the reply
3. Allow querying the checksum by code ID

which is implemented as follows:

cosmwasm

• Add WasmMsg::Instantiate2 (Add support for MsgInstantiateContract2 #1436)
• Implement instantiate2_address (Implement new predicable address calculation #1437)
• Remove msg argument (Remove msg argument from instantiate2_address #1554)
• Query checksum by code ID (Add WasmQuery::CodeInfo to get checksum for code ID #1561)
• Add example usage of WasmMsg::Instantiate2 in some contract (together with instantiate2_address) (Add Virus contract #1564)

Add checksum field to ContractInfoResponse. This one might be tricky since it adds a required field to ContractInfoResponse::new(code_id, creator). As long as we don't have that, the contract doing Instantiate2 needs to know both code ID and checksum. (#1546)

wasmvm

Add ContractInfoResponse.Checksum (CosmWasm/wasmvm#376)

[webmaster128]

Interesting use case brought up by @assafmo: with this a contract A can instantiate and immediately execute a contract B. That would be cool to showcase with WasmMsg::Instantiate2 and instantiate2_address.

[webmaster128]

I think we're done here :)