fastTwoDiff

Returns the difference and exact error of subtracting two floating point numbers. Uses an EFT (error-free transformation), i.e. a-b === x+y exactly. The returned result 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