feat: add Intersperse function

[wfxr]

Intersperse inserts a value between each element of a slice. It can be very useful to join items using a common element. Inspired by rust intersperse iterator method.

s1 = []int{1, 2, 3,}
s2 := Intersperse(slice, 100)
is.Equal([]int{1, 100, 2, 100, 3, 100}, s2)