feat(abigen): include ethevent proc macro in abigen code gen workflow

[mattsse]

This integrates the EthEvent trait in the abigen code generation process.

Motivation

The EthEvent makes the output of the abigen macro more consistent for events, and allows for generating further helper methods, as mentioned in #228.

Solution

Instead of generating the methods name, signature and abi_signature for each event provided in the ABI, the EthEvent trait is simply implemented via a #[derive(EthEvent)] during the abigen workflow. In addition all structs that are declared in the human readable abi are generated aswell. If a solidity event contains variables with solidity structs as tyoe , the generated rust struct variables have the corresponding rust. This has currently a limitation where the build fails if a solidity event has an array of a struct as input, however gakonst/ethabi#1 should remove this limitation.

Open Discussion

• The generated event struct still use a Filter name suffix, I think this is not necessary and could be removed.
• To use the EthEvent methods as before, an import of the EthEvent is required, right now the generated contract module does not pub EthEvent, which would be breaking as well as the minor change from &'static str to Cow<'static, str> in EthEvent::name and EthEvent::abi_signature, I'd think that is negligible.

[gakonst]

Merged, thank you. Supportive of removing the Filter suffix from the struct definition. I'll mark this as a BREAKING change in the next release, no problem otherwise.