Improvements for `*ContractEvent` and `*ContractFunction` APIs

[fselmo]

What feature should we add?

I see some low-hanging fruit in contract interactions with functions and events.

• contract.myFunction.signature & contract.MyEvent.signature

  • We could use a clearer attribute to signify signature. Right now this is the same thing as abi_element_identifier, though I'm not sure off the top of my head if this always returns the signature.

• contract.MyEvent.to_topic() (.as_topic(), .topic?)

  • We could use an API that returns the event as a topic (w3.keccak(text=event_signature).to_0x_hex()). This would significantly improve subscribing to topics on contracts:

  w3.eth.subscribe(
      "logs", 
      {
          "address": my_contract.address,
          "topics": [
              my_contract.events.MyEvent.to_topic()
          ]
      }
  )

[reedsa]

@fselmo I think you addressed the topic in #3578, though I think we forgot to add in the docs. signature was added in #3540. I'll go ahead and create a PR to add the docs for topic but I think we can close this out once that's in.