Handle ambiguous event names in contracts

[reedsa]

What was wrong?

Closes #964, #1704

The Contract class breaks down if a contract contains overloaded event names. BaseContractEvent._get_event_abi() calls eth-utils get_event_abi() but if any contract event shares the same name there is a problem with initialization of the Contract class.

Currently only the event name is used to filter ABI events from the contract ABI, so an exception is raised for "multiple events are found" when _get_event_abi is called on the Contract or ContractEvent class.

There are inconsistencies between Contract Functions and Events. The APIs should be the same for each so that there are methods to retrieve the ABI on the ContractFunction or ContractEvent. Exception messages can be shared across both and incorporate the same behaviors when a Function or Event cannot be matched with the ABI.

How was it fixed?

Add _get_abi methods to both BaseContractFunction and BaseContractEvent.

Modify get_abi_element to allow argument_names as an argument.

ContractEvent and ContractFunction initialized with an abi_element_identifier and abi. The abi_element_identifier stores the string signature representation of the event or function.

Also refactored _mismatched_abi_error_diagnosis so that messaging is clear for all ABI types, not just functions. MismatchedABI error messages are created based on the number of matches for specific filters. Error messages for MismatchedABI exceptions were using "encoding" terminology when the argument values were not encodable. Now that lookups can be made using the argument names and types (rather than values), the messages now state the arguments do not match the ABI.

Todo:

• Clean up commit history
• Add or update documentation related to these changes
• Add entry to the release notes

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