String::contains()

"abcd".contains("bc") //-> True
"abcd".contains("x") //-> False

public method contains(this: String, substring: String): Bool {
  if substring.len() > this.len() {
    return False
  }

  let i = 0
  while i <= (this.len() - substring.len()) {
    let section = this.substring(i, i + substring.len())
    if section == substring {
      return True
    }

    i += 1
  }

  False
}