Add support for events

[DaniPopes]

• SolEvent trait
• sol! support, similar implementation to error.rs

[prestwich]

this is more annoying than it seems, as the decoding is dependent on whether an indexed type is a dynamic. if dynamic, the hash of the encoded type is in the indexed field

this means that you can never just decode a topic. you have to determine whether the topic is a hash of a type that is found within the body