Cuts out the first appearance of the string sep from this string, in other words,
returns a tuple of two strings and a bool, the first string is the substring before
the first appearance of sep, the second string is the substring after the first
appearance of sep. The bool result is true iff sep appears in this string.

If sep does not appear in this string at all, return this string as the first string,
the empty string as the second, and false as the bool.