Fix Fraction.add and subtract for operands not in lowest terms

[lenamonj]

Fraction.addSub runs Knuth 4.5.1 on the operands as given, but the algorithm assumes lowest terms and getFraction(50, 100) is stored as 50/100. A shared factor inside an operand survives into the common denominator, so add and subtract throw ArithmeticException while the exact result fits an int (1073741823/2147483646 plus 3/5 is 11/10) and return unreduced results against the reduced form the javadoc promises. #1769 fixed the same shape in multiplyBy.

Both operands are now reduced before the Knuth step, as multiplyBy does. Operands already in lowest terms take the identical path, so the existing overflow and identity assertions hold.

Test: testAddSubtractUnreducedOperands, red on master and green with the change. Default Maven goal green.

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