Add query to determine if address contains a contract

[ethanfrey]

Resolves a large part of #893

Sometime a contract wants to know if it is called by a pubkey (external user) or another contract. Or if we are given a cw20-contract address, we can quickly check if it is really a contract or someone faking this.

Let's add is_contract(&self, addr: &str) -> bool on QuerierWrapper, such that it returns true iff addr is a valid address and there is a cosmwasm contract stored at that address. Add appropriate variant to QueryRequest and wasmd as well.

@webmaster128 what do you think of this? May be small enough to fit in 0.14? Just need to figure out where to place it. wasmd implementation should not be hard.

This is somewhat related to ideas in #143. Maybe we want to provide more info than just bool? We can also push to 0.14.1 or later if you think it is worthwhile to return more info and we should reflect on that some

[webmaster128]

I'd implement more or less this query into x/wasm: https://github.com/cosmos/cosmjs/blob/v0.25.0-alpha.2/packages/cosmwasm-stargate/src/cosmwasmclient.ts#L288-L305. Instead of throwing we turn the result into a Option<ContractInfo> or something like that. This would be well in line with the wasm query service https://github.com/CosmWasm/wasmd/blob/master/proto/cosmwasm/wasm/v1beta1/query.proto#L14-L55

[webmaster128]

I think this can already be implemented in contracts today by using a copy of this code, an arbitrary key and handling SystemError::NoSuchContract instead of converting it to an StdError: https://github.com/CosmWasm/cosmwasm/blob/v0.14.0-beta4/packages/std/src/traits.rs#L230-L261

[ethanfrey]

Hmmm... maybe we will want this Option<ContractInfo>.
With #889 we start caring about the admin of a contract more and more.

[ethanfrey]

Closed by #1089