(Vcharacter_composition_table): Don't define if external-DB feature is

(Vcharacter_composition_table): Don't define if external-DB feature is
supported.
(Qcomposition): New variable.
(Fget_composite_char): New implementation for external-DB support.
(Fput_char_attribute): Use `composition' property of each character
instead of `Vcharacter_composition_table' to store
character-composition rules if external-DB feature is supported.
(syms_of_chartab): Add new symbol `composition'.
(vars_of_chartab): Don't setup `Vcharacter_composition_table' if
external-DB feature is supported.