Refactor the ethereum_events.rs (Ethereum event parser).

[dzmitryhil]

Refactor the ethereum_events.rs code. Now, most of the methods have code duplication and parse the events in a weird manner one attribute by one (incrementing the next byte index), which makes the support extremely hard, because the change of the first lines of the code significantly changes the following code.

The refactored code should either use ABI-generated code to parser the events or a number of functions to parse the events attribute by index, like here.
Also, it should validate and return the details errors if case any of the sub-decoding functions return an error.

All methods of the "ethereum_events.rs" must be covered by unit tests. The input data might be taken from the solidity tests.

This issue can be done from the "native-reward" base branch.

Ethereum ABI docs