Neo-boa

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- Add ``Neo.Transaction.GetWitnesses``, ``Neo.Witness.GetInvocationScript``, ``Neo.Witness.GetVerificationScript``
- Fix ``IsPayable``
- Fix CI test wallet build issue

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- ``module.to_s()`` method now returns a string instead of printing it
- Update to allow ``CALL_FUNCTION`` opcode within a ``GET_ITER`` instruction.

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- Implement NEP8 call functionality

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- Use reduced sized fixtures for quicker tests

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- Python 3.7 compatibility

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- Updated NEP5 sample
- Updated equality operator conversion