ABI Generation

[liamcurry]

Hand-written ABI files are unnecessary and expose developers to risk if they aren't kept up-to-date.

Since we already have #[eosio_action] and #[eosio_table] attributes, it should be fairly straightforward to implement this feature by detecting these attributes and generating a JSON file.

[renlulu]

Thanks for your excellent job, and I will try to implement this future ：）

[liamcurry]

That would be great! Please let me know if you have any questions. Thanks.