[ucdn] Replace UCDN with a new UCD implementation

[ucdn] Replace UCDN with a new UCD implementation

UCDN was ~120kb of data.  New implementatoin is 69kb in default builds,
and 49kb if built with HB_OPTIMIZE_SIZE or __OPTIMIZE_SIZE__.  The
latter automatically enabled if built with -Os or -Oz.

There's room to shave off another 10kb or 20kb.  That will follow later.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1652