Allow contracts to query whether an address is a contract

[athei]

The following APIs should be introduced for contracts to call:

fn is_contract(account_id: AccountId) -> bool;
fn caller_is_origin() -> bool;

Technically, caller_is_origin might not be required as is_contract(self.caller()) could be used for that. However, caller_is_origin can be implemented without any storage lookup and therefore uses less gas. Sometimes it is useful to check whether a contract is called by another contract or the origin. caller_is_origin is a cheap way to do so.

Please note that when being called as RPC the origin can indeed be a contract just because the RPC allows the sender to be specified freely. This does not matter here because during on-chain execution the origin can never be a contract and therefore no interaction with dry-running can occur.

Does this API make sense to you @Robbepop @cmichi @ascjones ?

The issue came up here: use-ink/ink#804

[cmichi]

Yes, makes sense to me.

[athei]

Updated is_contract to be bool because tombstones no longer exist.