fastTwoSum

Returns the sum and exact error of adding two floating point numbers. Uses an EFT (error-free transformation), i.e. a+b === x+y exactly. The returned sum is a non-overlapping expansion (smallest value first!).

Precondition: abs(a) >= abs(b) - A fast test that can be used is (a > b) === (a > -b)

See https://people.eecs.berkeley.edu/~jrs/papers/robustr.pdf