Return a byte value of a character at point.
Optional 1st arg POSITION, if non-nil, is a position of a character to get
a byte value.
Optional 2nd arg STRING, if non-nil, is a string of which first
character is a target to get a byte value. In this case, POSITION, if
non-nil, is an index of a target character in the string.
If the current buffer (or STRING) is multibyte, and the target
character is not ASCII nor 8-bit character, an error is signaled.
(fn &optional POSITION STRING)