Checks if the string value inside this Opt is a substring of the given string.
The string to search in
A function that takes an Opt and returns an Opt
elemOfStrIn('abc')(opt('a')) // Some(true)elemOfStrIn('abc')(opt('d')) // Some(false)elemOfStrIn('abc')(none) // None
Opt.elemOfStrIn
Checks if the string value inside this Opt is a substring of the given string.