More flexibility to contract DSL, Async contract caller

[yglukhov]

One thing bugging me about contract dsl was the necessity to call call/send/exec over each invocation (e.g. let myValue = await myContract.getMyValue().call()). So I tried to fix that and here's what I came up with. Contract dsl is still async-and-web3-agnostic, but requires sender to define createMutableContractInvocation, createImutableContractInvocation and createContractDeployment functions, whose results are blindly passed to the caller. This allows to define async contract sender, while still keeping the old api around. See diff test_contracts.nim for demonstration of how new api looks like. With send/exec/call functions gone, the new way to adjust invocation options (like value, gas, gasPrice, etc) is the adjust macro, that creates a copy of async sender, adjusting its fields. E.g.

await myContract.adjust(value = 5.u256, gas = 10000).myContractMethod()

Any feedback is welcome.

[jangko]

This is great. Looks more natural and intuitive.
But honestly, I don't know much about the web3 usage and internals.
Need more eye on this, ping @arnetheduck @zah