Obtains an iterator pointing to
byte_index within the given line.
byte_index must be the start of a UTF-8 character. Note bytes, not characters; UTF-8 may encode one character as multiple bytes.
line_number is greater than or equal to the number of lines in the this, the end iterator is
returned. And if
byte_index is off the end of the line, the iterator at the end of the line is returned.
iterator to initialize
line number counting from 0
byte index from start of line
whether the exact position has been found