add keyword by or argument function f to set operations such as intersect

[johnnychen94]

The following is a demo for what I'm expecting: it returns pixels that have the same position regardless of color.

struct Pixel
    color
    pos
end

img = rand(T, 4, 4)
pixels = Pixel.(img, CartesianIndices(img))

intersect(pixels[1:3, 1:3], pixels[2:3, 2:4]; by=p->p.pos) # keyword
intersect(p->p.pos, pixels[1:3, 1:3], pixels[2:3, 2:4]) # function

Not very sure how broadly this is useful to other fields.