fix: typing for mixed-width sum whose left operand is narrower

[vbar]

Binary.__init__ typed the result of every arithmetic binary operation as left_variable.type, ignoring the right operand. Solidity implicitly widens mixed-width int/uint operands to their common (wider) type, so (a + b) should be typed uint256, not uint128. Because the temporary variable was mistyped as uint128, the using Lib for uint256 binding could not be resolved, LibraryCall.function was left None, and a later pass crashed with AttributeError: 'NoneType' object has no attribute 'parameters'.

Fixes #3103 .

[CLAassistant]

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