Implement a flipsign function

[eschnett]

I find the non-standard flipsign function https://docs.julialang.org/en/v1/base/math/#Base.flipsign convenient, e.g. to implement upwind finite differencing stencils. It can be defined as

flipsign(x, y) = copysign(1, y) * x

but can be implemented more efficiently as

flipsign(x, y) = x ^ (y & SIGNMASK)

[gquintin]

If you allow me, I will "merge" this issue with #92 to have all this operators at one place.