Add access to core Multicall contract methods

[DefiDebauchery]

The core Multicall contracts have various methods for accessing base attributes of the chain at the executed block number, the most important (IMO) being getEthBalance() and getCurrentBlockTimestamp().

getBlockNumber() is slightly redundant since it's already provided via aggregate() / tryBlockAndAggregate() (exposed via #74), but added, along with all other methods, for completeness.

One real-world example that I use in my own code:

calls = [
	Call(wrapped_native, ['balanceOf(address)(uint256)', my_address], [['weth_balance', None]])
]
mc = Multicall(calls, _w3=rpc)
mc.get_eth_balance(my_address)

results = await mc.coroutine()
// {'weth_balance': 4858579460227, 'eth_balance': 60178543536789}

The return signature can be overridden to a different key or formatter.

Edits to overall structure or nomenclature welcome.

[BobTheBuidler]

Could you add a test case for each of these new methods?

[BobTheBuidler]

just out of curiosity, why don't you query these from the w3 object directly? It should be faster for you

[DefiDebauchery]

just out of curiosity, why don't you query these from the w3 object directly? It should be faster for you

Not sure I understand; how/why would it be any faster than doing it from within the context of a query I'm already making?

As for the intent overall, it's more or less separation of concerns; the multicall module should handle everything related to making multicall queries, without me having to remember the address or signature.

Will add test cases as soon as I remember how to do that 😅 My least favorite part

[BobTheBuidler]

Sorry, that was an oversight. I was thinking "theres a specialized method for that on the rpc" but missed the fact that you're referring to the timestamp

If you get the tests added I'll setup a testing workflow so this PR and future PRs is tested right here on github