fix: attempt to encode uint with a negative value results in a crash

[JeneaVranceanu]

Summary of Changes

Update

The initial bug was caught by mistake by using older version of the web3swift library.
But nonetheless an encoding bug was fixed: passing negative value to ABIEncoder.encodeSingleType with a type .uint must return nil.
For example, web3js produces Uncaught Error: value out-of-bounds for such cases. We are returning nil for now.

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Attempts to encode a negative number with ParameterType .uint(...) resulted in a crash.
The fix is located in ABIEncoder.convertToBigUInt.

Test Data or Screenshots

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[janndriessen]

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