feat(abigen): extend ethevent trait methods and decoding

[mattsse]

Motivation

This follows up on #234 and also provides a solution for #98

Solution

The EthEvent trait is extended with a method to create an Event builder directly (closes #234).
The EthEvent proc macro now derives a decode_log(log: &RawLog) that parses the log message into an instance of the type it's derived on.
This introduces another trait EthLogDecode which only purpose is to create a common RawLog -> Self interface for all the generated EthEvents of a contract and the generated enum wrapper that is generated for events that have more than 1 event in their abi (closes #98)

Example

if a contract is created as follows

abigen!(
    SimpleContract,
    r#"[
    event ValueChanged(address indexed author, string oldValue, string newValue)
    event OwnerChanged(address indexed author, address indexed author)
    event Log(string msg)
]"#
);

an enum

pub enum SimpleContractEvents {
    LogFilter(LogFilter),
    OwnerChangedFilter(OwnerChangedFilter),
    ValueChangedFilter(ValueChangedFilter),
}

is created.
In addition, if the abi has at least one event the generated contract gets a new method events(&self) -> Event<M, Events>, where Events is either the sole event for contracts with only a single event or the generated enum for contracts with more than one event.
In the latter case, the protocol decoding works in such a way that all variants are tried to be decoded one after the other and the first successful attempt is returned

Breaking changes

• Renamed the Contract::event(&self, &str) -> Result<Event<_>,Error> to event_for_name and introduced Contract::event<T:EthEvent>(&self) -> Event<_> + another convenience function

[gakonst]

1 comment as discussed on the fully qualified syntax, otherwise looks great to me!

[gakonst]

Having fully qualified syntax here requires that we have to declare ethers_providers/core/contract in the consuming crate's cargo toml. It'd be ideal if we avoided that. Could you please revert this change?

[mattsse]

Instead of removing the fully qualified syntax, I simply renamed the top level imports to their crate name in e018a70, which achieves the same effect, plus has the benefit, that the EthEvent trait, which resides in ethers_contract is usable with ethers_contract as crate dependency.

[gakonst]

Awesome, thank you, merged.