Binary Sum

Write a function that takes two binary strings and returns their sum (also a binary string).

('0000101', '0000010') -> '0000111';
('0000001', '0000001') -> '0000010';
('111111', '000001') -> '1000000';
('111111', '111111') -> '1111110';
('111111', '000000') -> '111111';

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