Implemented char.is_(lead|trail)_surrogate/1.

For save conversion from UTF-16 to UTF-8, it is essential to know
if a character is a lead or trail surrogate character.
It is implied that
char.is_surrogate(Char) <=>
    ( char.is_lead_surrogate(Char)
    ; char.is_trail_surrogate(Char)
    ).

library/char.m:
    see above.