In Lua, write coroutine substrings function. This takes a single parameter s, which must be a string. It yields all substrings of s: first the unique length-zero substring, then all length-one substrings, and so on, ending with s itself.

For example, if substrings("abca") is called as a coroutine, then it should yield the following, in order:

"" "a" "b" "c" "a" "ab" "bc" "ca" "abc" "bca" "abca"