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.