Find the position of a single non-matching character between two strings

[stevengj]

@rocco_sprmnt21

Regarding correctness, I point out that a “simpler” solution was required to the problem of finding the position of a SINGLE non-matching character, not the first of many non-matching characters.

It also fails this test:

julia> s1, s2 = "zzzz", "zzzx"
("zzzz", "zzzx")

julia> findfirst(==(only(setdiff(s1,s2))), s1)
ERROR: ArgumentError: Collection is empty, must contain exactly 1 element